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Control of Algae in Planted Aquaria

Paul L. Sears, Ottawa, Canada,
Kevin C. Conlin, Montreal, Canada,

March 1996

Reproduction of this document by any means for commercial purposes requires the express written consent of the authors. Copyright 1996



Experiments with planted aquaria appear to indicate that growth of green algae, red algae, and cyanobacteria is suppressed in planted tanks in which the availability of phosphate is the factor limiting plant growth. It is believed that when light, CO2, N, K, and all micronutrients and trace elements are present in slight excess relative to the amount of phosphate available for plant growth, certain higher plants are able to out-compete algae and cyanobacteria for the phosphate in the water column, starving them of this essential nutrient. Two case studies are presented as evidence for this hypothesis.


There are few things as frustrating to the aquarist interested in growing aquatic plants as algae. After spending a small fortune on lights, substrate additives, liquid fertilizers, and CO2 systems in an attempt to get good plant growth, the aquarist is often rewarded with a lush carpet of algae. Unsightly and stubbornly resistant to eradication, the algae destroys the aesthetics of the tank while limiting plant growth by competing with them for light and nutrients.

In desperation, the aquarist experiments with various forms of algae control, including algicides, bleach dips, antibiotics (for cyanobacteria), physical removal, and the introduction of an assortment of algae-eating fish and invertebrates. Feed levels are reduced, light duration is decreased, and various combinations and amounts of fertilizer are tried, until through trial and error an uneasy truce is reached.

In the search for a solution, the aquarist is faced with an almost complete absence of information as to which of the many tank parameters should be altered in which order to eradicate algae already present while still maintaining favorable conditions for plant growth. This is hardly surprising given the huge number of variables, including light strength, duration, and spectrum; CO2, micronutrient, macronutrient, and trace element concentrations; fish load; plant and algal species and density; and water chemistry and temperature. Sometimes the information that does exist appears contradictory; in [1], excessive growth of cyanobacteria is attributed to high nitrite and nitrate levels, yet this pest is often seen in fully cycled aquariums with no measurable nitrite or nitrate at all.

One option available to aquarists with deep pockets is to follow the proprietary Dupla system [2], a system of liquid fertilizer drops, tablets, tap water conditioner, substrate additive, and undergravel heating coils. Magnificent planted aquaria are routinely produced this way, but the components are expensive, the ingredients are not disclosed on the package (but see [3]), and little insight is gained into the relationship between plants and algae (or how the system should be "tweaked" for best results).

Like many others, the authors attempted to grow aquatic plants using typical aquarium configurations and various commercial liquid fertilizers and substrate additives. Frustrated by their inability to attain results even remotely resembling the photographs in the literature, they began systematically to add specific nutrients to their tanks and record their observations. Although eradication of algae was not the immediate goal of the experiments, it was noted that once the aquarium water was supplemented on a daily basis with trace elements, micronutrients, and the macronutrients K and N but not P, not only did the plants begin to grow extremely well, algae of all types began to die off rapidly.

In this paper, case studies of the authors' aquaria are presented. The case studies are followed by a discussion of the results in which a number of hypotheses are considered. These hypotheses are quite testable, and it is hoped that other hobbyists will be willing to perform controlled studies on their aquaria to either support or disprove them.

Case Study #1

Initial conditions as of November 1993: 500L aquarium with undergravel and canister filters; 240W fluorescent lighting, 12 hours per day; 15W UV sterilizer; 8cm 2mm gravel with a few laterite balls; no CO2 additions; no fertilizer; about 40 3-12cm fish; water temperature 27C, pH 7.5, GH 100ppm, NO3- 50ppm, 25% change every week; planted mainly with Hygrophila polysperma and Vallisneria gigantea, with a few Echinodorus sp., Cryptocoryne sp., and others.

The aquarium was purchased second-hand as a complete set-up and had been in operation at least six months prior to being acquired by the author [Conlin]. About a month after after being moved to the author's residence, a dense coat of green algae developed on the gravel-coated glass-fiber backdrop. Plant growth was marginal, even for the H. polysperma, which had small 3cm leaves and was not spreading. Hygrophila difformis was introduced and promptly lost its lower leaves.

Change: Twenty Terrapur cones were embedded in the substrate and Sera liquid fertilizer was added as directed to the tank water during water changes. Hydrocotyle leucocephala was introduced.

Effect: Growth of H. polysperma, H. difformis, and V. gigantea improved but long strands of green thread algae started growing on the backdrop. Various Echinodorus and Cryptocorynes showed marginal growth. The H. leucocephala quickly degenerated, leaving a few small fragments growing at the surface. Some red algae was noted on the leaves of Anubias barteri var. nana and along the leaf margins of the V. gigantea. After a few months, blue-green algae (cyanobacteria) began to cover the gravel and some plants.

Change: Erythromycin sulfate was added to the water at approximately 3.2mg/L.

Effect: Cyanobacteria disappeared for several weeks but eventually returned.

Change: Less food (particularly frozen bloodworms) was offered to the fish and a DIY yeast CO2 system was connected to the tank.

Effect: Cyanobacteria remained. Nitrates were unmeasurable. Plant growth was noticeably faster. Depending on the state of the yeast reactor, tank pH varied from 6.8 to 7.5.

Change: The Sera fertilizer was eventually discontinued on the assumption that it was contributing to the growth of the cyanobacteria. It was replaced with a commercial iron-containing trace element mix (initially 1/8 tsp of powder a day, soon increased to 1/4 tsp a day).

Effect: Nitrates rose to about 20ppm. Green algae began to replace the blue-green algae on the plants and gravel. An iron test kit indicated the presence of iron at a concentration below the first level on the color chart (0.25ppm). Plant growth accelerated, but the leaves on the H. polysperma became bent and the lower leaves fell off. This was assumed to indicate a potassium deficiency [4].

Change: K2SO4 was added to the tank at the rate of about 1/4 tsp/day.

Effect: Shortly thereafter the nitrate level became unmeasurable, leading the author to conclude that nitrogen was now the factor limiting plant growth.

Change: KNO3 joined the list of fertilizers being added to the tank on a daily basis. To simplify dosing, the trace elements, K2SO4, and KNO3 were incorporated into a liquid fertilizer. The mixture was adjusted to keep the nitrates at about 10ppm when the enough liquid was added to the tank (about 12mL) to keep the iron at an estimated 0.1ppm.

Effect: At this point, growth of the H. polysperma, H. difformis, and V. gigantea became exceptional, requiring weekly trimming. Somewhere along the line, duckweed had been introduced to the tank and it now began to clog the surface. Cryptocorynes and Echinodorus began growing new leaves every few days and sending out runners. Algae of all sorts quickly declined to the point where careful observation was required to find it. Strangely, the Echinodorus were unusually pale in color despite iron fertilization. Magnesium deficiency was suspected.

Change: Epsom salts were added to the fertilizer mix.

Effect: Within a few days, new Echinodorus leaves showed normal coloration.

Change: The yeast CO2 system was upgraded to a constant-flow tank/regulator/needle valve system.

Effect: Reduced pH swings (6.8-7.0). More free time for the author.

Change: After several months, during which plant growth remained excellent and algae scarce, four pellets of "Vigoro Super Triple Phosphate 0-48-0" (almost certainly Ca(H2PO4)2) were added to the tank as an experiment (approximately 0.1ppm phosphate).

Effect: The next day green spot algae was observed on the glass and Echinordorus leaves, followed two days later by blue-green algae that grew on some plants and driftwood. Duckweed soon required daily removal. Nitrates were unmeasurable several days after the phosphate was introduced but returned to 10ppm a week or so later (sadly, they weren't measured just before adding the phosphate). Two weeks after the experiment began, the blue-green and green-spot algae began to decline, and duckweed growth returned to normal.

Current status: Plant growth remains excellent. Some traces of algae still remain, principly green spot algae.

Case Study #2

Initial conditions as of May, 1994: 160 L tank, 12 cm of 3 mm gravel with 1.7 kg of Terralit in the bottom 3 cm. Canister filter with carbon, 80W of cool white fluorescent light, CO2 fertilization, very small fish load (6 flame tetras). Water hardness approximately 120 ppm CaCO3 equivalent, pH ~7.0, temperature 25C, 25% change every few days.

Plant growth was slow, and brown algae that appeared to be a form of cyanobacteria (rapid growth in sheets, easy to remove) grew on the plants and substrate. Attempts to control the algae by frequent water changes and mechanical removal were ineffective. All water changes were accompanied by disturbance of the top 1 cm of the substrate.

Change: A potassium/iron fertilizer was added (0.9 ppm K, and 0.06 ppm FeIII) to the replacement water at water changes. The fish load was increased to 23 flame tetras (6 adult, 17 juvenile) and six otocinclus. The cool white lights were replaced with inexpensive plant tubes.

Effect: No change noted.

Change: K/Fe addition was stopped, and plant tablets (10-14-8) were inserted into the substrate in small pieces near plant roots. A total of 35g of tablets was added over a period of few weeks.

Effect: Some improvement in plant growth was observed. Unicellular green algae proliferated, reducing the visibility in the water to as low as 25cm. Frequent water changes had little effect on the algae.

Change: Fritz Super Clarifier (active ingredient(s) unknown) was added as directed to the tank water.

Effect: The unicellular algae became trapped by the filter. Because a recurrence was expected if the aquarium parameters were not altered, another change was made immediately:

Change: Addition of trace elements (homebrew formulation of Fe, Mn, Cu, Zn, B, Mo, and EDTA) with potassium sulphate at water changes. The dosage was computed to give 0.1ppm iron and about 1ppm potassium in the replacement water. Carbon was removed from the filter.

Effect: Plant growth improved, but blue-green cyanobacteria appeared and began to spread. Nitrates were found to be unmeasurable.

Change: Addition of potassium nitrate began in 1-2ppm NO3- doses, initially once every 5 days, increasing to daily once the author [Sears] became convinced of its lack of toxicity at these concentrations. Potassium sulphate, previously added to replacement water, was now dosed with the potassium nitrate at about 1-2ppm K. A commercial trace element mix (composition given in Appendix A) replaced the homebrew formulation. Magnesium sulphate addition was begun shortly after at a concentration of about 0.25ppm Mg.

Effect: Significantly better plant growth, but patches of cyanobacteria continued to grow on the plants and substrate. Green thread algae appeared on the brightly lit parts of plants. It was found that nitrate introduced to the water in 1-2 ppm doses was not detectable one or two days later.

Change: More plants were added. In the process, several old plants were uprooted, exposing the buried fertilizer tablets to the water.

Effect: Increase in green algae and blue-green cyanobacteria.

Change: Disturbance of the gravel at water changes stopped. Specifically, gravel vacuuming was discontinued, and replacement water was poured into the tank gently. Since the substrate evidently still contained considerable phosphate in the form of undissolved fertilizer tablets, it was thought best to disturb it as little as possible.

Effect: Algae of all types declined rapidly. It no longer appeared on the leaves of fast-growing plants, and apparently died and fell off the older leaves of slower-growing plants.

Change: Reduction of hardness of water to 60 ppm CaCO3 equivalent. This resulted in a drop of pH to approximately 6.7 (which was the reason for the change), and a temporary jump in the iron concentration in the tank, from less than 0.2 ppm to 2 ppm.

Effect: All Cryptocoryne sp. in the aquarium lost some leaves. Algae continued to decline.

Current status: All of the plants in the tank are growing well, including the Cryptocorynes that lost their leaves. Stem plants require weekly trimming, and floating plants need thinning every few days. The only algae in evidence are some small patches of cyanobacteria on the substrate and a little green algae on brightly lit parts of the Vallisneria gigantea, the Cryptocoryne balansae and the Bacopa caroliniana. Disturbance of the substrate (for replanting of cuttings) has led to minor algae outbreaks (green algae if the nitrate concentration is at least few ppm, cyanobacteria otherwise). Small amounts of (apparently dying) material are still in evidence on some of the oldest Anubias barteri var. nana leaves. The water change frequency has been reduced to 25% every two weeks.


The observations in the case studies are consistent with the following hypothesis: when light, CO2, N, K, and all micronutrients and trace elements are present in slight excess relative to the amount of phosphate available for plant growth, certain higher plants in the aquaria are able to out-compete algae and cyanobacteria for the phosphate in the water column, starving them of this essential nutrient.

Exactly why higher plants should be able to outcompete algae for phosphate is unclear. Perhaps their roots give them some advantage, or they simply need much less phosphate than algae to thrive. Nor is it known which of the many plants in the test aquaria are responsible for stripping the water of phosphate, although the fast-growing duckweed and stem plants with roots growing above the substrate (notably Hygrophila spp.) are likely culprits. That phosphate is the factor limiting the plant and algae growth in the test aquaria has been reasonably well established; it is the only known plant nutrient not added to the 500L tank in any form other than fish food, and deliberately adding concentrated phosphate to this tank induced almost immediate algae growth (and a rapid duckweed explosion too). Since the plants continue to grow very well, they are clearly gaining preferential access to whatever phosphate is available. There may be some literature unknown to the authors that offers an explanation. If not, it should be fairly easy to conduct controlled experiments with a sensitive phosphate test kit and a few spare tanks containing only algae, one or two plant species, and nutrients. An experiment that shows that duckweed thrives at phosphate concentrations as low as X ppb, but green algae and cyanobacteria require significantly more than X, would offer strong support for the hypothesis.

According to the hypothesis, If the higher plants are unable to utilize all of the phosphate present in the water column because of a deficiency of some other nutrient, algae will thrive. The type of algae appears to depend on the availability of other nutrients. In the test aquaria, it was found that when nitrates were unmeasurable, cyanobacteria predominated. It is suspected that nitrogen deficiency favors the growth of cyanobacteria because these organisms can fix the atmospheric nitrogen dissolved in the aquarium water. When nitrates were available, green algae predominated. Some red algae was also observed in the 500L tank before CO2 fertilization was introduced. Because others have observed that tanks with CO2 fertilization have relatively little red algae [5], it tempting to speculate that at least some red algaes are able to utilize bicarbonate, giving them an advantage in aquaria where most of the available carbon is in this form (typically those with high carbonate hardness and high pH). The following paragraph summarizes the apparent relationship between nutrients, plants, and algaes:

If the aquarium is P limited, higher plants will outcompete algaes of all types for P, and the algae will disappear. If not, and N in the form of nitrates and ammonia is deficient, cyanobacteria will thrive, otherwise green or red algae will predominate. Red algae is favored over green algae if most of the available carbon is in the form of bicarbonates.

The factors that determine which species of algae will predominate in a given situation have obviously been greatly simplified. In [5], for example, nitrate concentrations in excess of 30ppm are claimed to be detrimental to the growth of green algae but not to cyanobacteria, so one would predict that cyanobacteria would predominate at high nitrate levels.

There is a tradition in the hobby of using fish food (usually processed by the fish first) as the source of all macronutrients for the plants in an aquarium. When this is done, it appears that first K and then N become the factors limiting plant growth (i.e. there is insufficient K and N in the food relative to the amount of P, at least for the fish foods the authors use). Thus, supplementary K and N must be added or free phosphate will be available to fuel algae growth (this contradicts the prevailing wisdom in the aquarium hobby that one of the ways to reduce algae growth is by reducing fertilization; in fact, additional nutrients are required). Other alternatives are to restrict feeding to the point where the growth of algae due to unused P is tolerable (another common piece of advice), an approach likely to result in poor plant growth due to nutrient starvation, or to use a phosphate-removing resin.

Some of the plant species in the 500L tank grow very slowly compared to the same species in the 160L tank (Echinodorus sp. in particular). The 160L tank has an enriched substrate with no deliberate water circulation, whereas the 500L tank has a relatively inert substrate with a 300gph UGF. It is highly unlikely that all plants are equally adept at extracting phosphates directly from the water column, and it appears that the fast-growing plants in the 500L tank are depriving the other plants of this nutrient which (thanks to the UGF) is distributed evenly throughout the tank. Slow-release phosphate tablets will be placed around the roots of these plants to see if growth improves. Both authors agree that the substrate design of the 160L tank (solid fertilizer at the bottom of an inert substrate) gives the better results, probably by making phosphate more-or-less equally available to all plants without allowing too much to leach into the water column where it is available to algae.


Despite the lack of controls on the various experiments, and the inability of the authors to directly measure phosphate in the aquaria, there is compelling evidence to support the hypothesis that all types of algae (including cyanobacteria) can be effectively controlled in planted aquaria by ensuring that phosphate is the factor limiting plant growth. In two aquariums with different volumes, substrates, lighting, and plant, algae, and fish populations, effective control of algae was achieved by enriching the tank water with CO2, micronutrients, trace elements, N, and K. Despite high initial algae loads, these tanks are now almost free of visible algae and have remained so for several months. Furthermore, in the 500L tank it was shown that phosphate limiting was occuring by adding phosphate to the tank water and observing the almost immediate growth of green spot algae and cyanobacteria. It has also been shown in the 160L tank that disturbances to the phosphate-containing substrate result in algal growth if there is significant (more than approximately 1 ppm) nitrate in the water, and in growth of cyanobacteria if nitrate is not present at this level. It is important to note that plant growth in both tanks is excellent, so algae control has not been achieved at the expense of the plants.


Plants cannot grow without phosphate. However, in order to keep a planted aquarium relatively algae free, free phosphate in the water column must be minimized. The following recommendations will help achieve this goal:

(a) A slight excess of light, CO2, K, N, micronutrients, and trace elements should be maintained to allow the plants to utilize all of the available phosphate. The authors recommend the following:

Since inexpensive tests are not available for trace elements, micronutrients, or K, these items are dosed as some percentage of the measurable nutrients. The authors have had considerable success with mixtures that duplicate the relative concentrations present in Tropica Master Grow fertilizer [6]. For those readers wishing to "roll their own", a balanced fertilizer recipe is given in the Appendix. Various commercial aquatic plant fertilizers are also available, but it may be necessary to purchase several products to ensure complete nutrient and trace element coverage. Daily dosing is highly recommended because it may prevent temporary nutrient depletion, which could make phosphate available on an intermittent basis and prevent the algae from starving.

As a general approach to optimizing plant growth and reducing algae, the following procedure is suggested:

  1. Set the light and CO2 levels.
  2. Add an iron-containing trace element mix (preferably one that already has Mg) to the tank every day, adjusting the quantity on a regular basis to achieve the target iron level. For mixes without Mg, add Epsom salts as well in the ratio of about 1.5-5.0ppm Mg to 1ppm Fe.
  3. A week or so after reaching the target Fe level, check the nitrate level. If nitrates are below about 2ppm, proceed to the next step. Otherwise, add enough K2SO4 to the tank every day to drop the nitrate level to as close to zero as possible and keep it there (if the nitrates don't drop, then something other than K is limiting plant growth and some detective work will be required to find it). Incidentally, measuring the nitrate level is helpful for general tweaking; if adding nutrient X causes the nitrate level to drop, then the tank is probably deficient in X.
  4. Add enough KNO3 to the tank every day to get a 3-5ppm nitrate reading (one of the authors [Conlin] obtains satisfactory results with 10ppm).

Once the relative amounts of trace elements, K2SO4, and KNO3 have been determined, it becomes a simple matter (if desired) to concoct a liquid fertilizer that can be poured into the tank each day. Using a mix of dry powders is not recommended as powders tend to separate.

The procedure just described ensures that there will always be a slight excess of nitrogen in the tank. Some terrestrial plants will not flower if nitrogen is abundant, and this may be the case for some aquatic plants too. It would be an interesting experiment to withhold fertilization for several weeks after a lengthy period (say 6 months to a year) of good plant growth to attempt to induce flowering.

There is a possibility that some of the trace elements will accumulate over time to levels toxic to plants if regular water changes are not done. 25% water changes every second week should prevent this from happening.

(b) Grow fast-growing plant species that can efficiently extract nutrients directly from the water column. These plants will rapidly strip phosphate from the water, making it unavailable to algae. Floating plants (Lemna minor, Limnobium laevigatum) and stem plants that grow roots at internodes (Hygrophila sp.) are suggested for this purpose.

(c) Enriched substrates are probably the best means of supplying phosphates to plants provided steps are taken to minimize the leakage of phosphate into the water column. Substrate fertilizers such as Pond Tabs should be buried deep in the substrate where their nutrients are preferentially available to plant roots. Substrate circulation should be minimized to prevent phosphate from leaching too rapidly into the water column. Avoid gravel cleaning and other substrate disturbances if at all possible. Eliminating substrate circulation completely would not be desirable (even if it were possible) because supplementary fertilizers are usually added to the water and must be transported to the roots somehow.

(d) There will always be some residual algae in a planted tank because it is impossible to keep the water completely phosphate free. The amount of residual algae will be very small, but a good selection of algae-eating fish (Otocinclus sp., Farlowella sp., Ancistrus sp., Crossocheilus siamensis) and invertebrates (Caridina japonica shrimp and some snails) is desirable anyway for controlling the algae outbreaks that occur when the tank is first set up, the substrate is disturbed, or the nutrients are incorrectly dosed.

(f) Do not use phosphate buffers to control pH. Use of these buffers may produce phosphate concentrations as high as 100ppm, almost certainly resulting in very impressive algae blooms.

(g) Algicides such as simazine and copper are not recommended because they damage plants and may be unhealthy for fish as well [7][8].

(h) Miscellaneous considerations:

Tap water is not recommended as a source of trace elements because it may be deficient in one or more elements, and rapid plant growth is likely to deplete the elements far more quickly than they can be replaced.

Certain water treatment products (Aquasafe, NovAqua) should be avoided as they bind metals (including iron), making them unavailable to plants. They may also contain phosphate buffers. Simple dechlorinators or products such as Amquel are a better choice for treating tap water during water changes.

Carbon filtration may remove necessary trace elements from the water. With regular water changes and good plant growth, carbon filtration is not necessary and should be omitted.

(i) As a general principle, avoid adding fertilizers, water treatments, or any other products to one's aquarium unless the products completely disclose the concentration of each ingredient present. Otherwise, there is no way to knowing what effect (if any!) these products will have on the aquarium's inhabitants.


The authors would like to thank Ed Tomlinson for running various experiments on his tanks on our behalf. Various participants in the Aquatic Plants internet mailing list (too numerous to list here) have contributed many useful observations and insights. Finally, the efforts of the reviewers, Dave Huebert and Karen Randall, are greatly appreciated.


[1] Baensch, H. and Riehl, R. Aquarium Atlas, Tetra Press, 1987.

[2] Horst, K., and Kipper, H. The Optimum Aquarium, AD aquadocumenta Verlag GmbH, 1986.

[3] Booth, George "[F][plant] CARBON as a SUBSTRATE", rec.aquaria newsgroup, 8 Aug. 1994 (also available on the Web).

[4] Frank, Neil "Nutrient Deficiency Symptoms"

[5] Baensch, H. and Riehl, R. Aquarium Atlas Volume 2, Tetra Press, 1993.

[6] Christensen, Claus "Re: Tropica Fertilizer", Aquatic Plants Digest V1 #165, 5 July 1995.

[7] Frank, Neil "Chemicals to Control Algae - The Use of Simazine", The Aquatic Gardener, Vol. 4 no. 6, 1991 (also available on the web).

[8] Gargas, Joe "Chemical Treatment of Ectoparasites Afflicting Fish Part I", Freshwater and Marine Aquarium, Oct. 1993.

Appendix A - Fertilizer Recipe (Poor Man's Dupla Drops)

(Most of the ingredients can be purchased at hydroponics shops or garden supply stores. Epsom salts are available inexpensively at pharmacies)

Dissolve the trace element mix in 150mL distilled water, then add the remaining ingredients. Pour in additional water to make 300mL solution. The HCl helps prevent the growth of fungus and may be omitted if the mix is kept in the refrigerator. Add enough mix to the tank every day to keep the Fe level at about 0.1ppm (the exact amount will have to be determined by experimentation, but 3mL per 100L tank water is about right for a tank with rapidly growing plants). Measure nitrate levels regularly, and adjust the amount of KNO3 in the mix to maintain 3-5ppm (this step is fairly important). Those concerned about adding nitrates to their aquarium can dose the KNO3 separately, omitting it initially and adding it later as required to obtain the desired concentration.

The shelf life of the solution is unknown. Make small batches, or store only dry powders (but mix them with water before adding them to the aquarium).

If test kits are not available, satisfactory results can be obtained by adding 1mL mix to 10L replacement water during water changes.

Here are some comments from the Aquatic Plants Mailing List.

The original source for this document may be found c/o Ed Tomlinson
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