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The substrate in a plant tank has three purposes. First and foremost, it provides a source of nutrients for the plants via adsorption through the plant roots. Second, it provides an anchor for the plants. Third, it provides an attractive and natural looking background for both the plants and fish.

There are some things the substrate should not do in a plant tank. For one thing, the substrate should not alter the water chemistry in the aquarium. Some types of commonly sold aquarium gravels contain limestone or calcium and can lead to uncontrolled hardening of the water. This may be fine for African cichlids, but is a severe problem for the plants and fish typically found in heavily planted tanks.

Another important factor in selecting the substrate material is the grain size. It should be selected such that it is neither too fine nor too coarse. Too fine a material will tend to become solid over time, due to biological waste material acting as a "mortar" between the substrate grains. As reported in "The Optimum Aquarium", this is a long process, taking on the order of many years to become a problem. The overall effect is to prevent nutrients from reaching the plant roots.

Gravel that is too coarse has two obvious problems. First, it will not offer a very good foothold for the plants, especially plants that have delicate roots. The second is that excess food and fish waste can settle into the rather large gaps in the gravel, making it difficult for scavengers to get to it. This leads to excess nitrate buildup as the material decomposes and leads to water quality problems in the long run.

Fortunately, the most common kind of aquarium gravel available in the fish shops meets all these criteria. We selected a common quartz gravel sold as "sandblasting gravel". The brand we bought is named "TexBlast" and is light brown in color with a grain size of between 1/16" and 1/8" (2-3 mm).

The SST has 3 1/2" of gravel, amounting to 150 pounds or 3 bags. The amount of this gravel needed for a tank can be calculated by determining the volume of gravel needed (in our case 47" x 17" by 3.5") and dividing by 19 cu. in. per pound.

Most plant books recommend that some additives be put in the substrate to enhance plant growth. Substances like peat, potting soil, sand and clay are often mentioned. "The Optimum Aquarium" recommends the use of laterite in the lower one third of the substrate. The book is lacking in specific reasons for using laterite, but leaves the impression that the main reason is to supply iron to the plant roots.

One clue to the real function of laterite and substrate heating is the claim that they "integrate the substrate into the aquarium". The following information is credited to Jeff Frank and provides more insight into what this may really mean. Hopefully, we haven't introduced too many errors in our paraphrasing of his comments.

Laterite is a remnant of volanic rock which has been highly weathered by exposure to tropical temperature, precipitation, and forest derived humic acids over geologic time. Laterite, or any clay for that matter, has a crystalline structure which has many negatively charged sites which are important for plant chemistry. Except for decomposed organic matter there are no negatively charged sites in the aquarium. Soils from temperate regions (clay fractions of which are relevant for comparision to the tropical laterite) not exposed to the accelerated wheathering of the tropics retain too much Ca++ and Mg++ which will adversely affect hardness and pH in a plant tank.

Many sources agree that ammonium is the preferred form of nitrogen for plant utilization. Ammonium, and many other positively charged ions like Fe++, K+, Ca++, Mg++, and Na++, are attracted by the negatively charged sites provided by the laterite.

The negative sites attract and hold the ammonium ions like a magnet until a plant root hair exchanges another positvely charged ion for the ammonium (adsorption) and takes it in to metabolize into amino acids, and ultimately protein. By providing this readily usable source of nitrogen, the additional benefit of removing ammonia (due to ammonia/ammonium equilibrium) is realized. Instead of just the "ammonia to nitrite to nitrate" cycle in biologic filtration, the "nitrogen cycle" and its accumulating nitrate levels is avoided altogether. The nitrogen ends up being removed from the tank as you cut and prune excess plant tissue because plant tissue is partly made of protein, which is 14% nitrogen.

The substrate heating coils are an important element in the substrate for a number of reasons. The primary reason is that the substrate immediately around and just above the cable is heated a few degrees higher than the ambient water and rest of the substrate. This provides a gentle convection current, bringing nutrients into the laterite area of the substrate where they can held and made available to the plant roots. Cables work well in this aspect, compared to a heating pad or other methods, since the spacing between the coils and being positioned slightly above the tank bottom provides a fireplace-like draft. The slow current is important because it better matches the rate at which reduction reactions occur (as compared to the flow created by an undergravel filter). This allows for a better utilization of the available nutrients.

Another reason is that for every 10 degrees higher temperature between 50 and 90 degrees F there is a 2-3x increase in the rate at which reduction reactions take place. This is important because iron and other nutrients are more soluble in their reduced state, that is, Fe++ is more available than Fe+++. With the extra heat in proximity to plant roots and the iron bearing laterite, optimum growing plants which would otherwise be Fe++ limited have a slow, but steady Fe++ source.

A third reason is that some plant books mention that having "warm roots" is beneficial to many plants, especially Barclaya longifolia. The heating cables provide a gentle source of warmth. We have noticed when planting the tank that the gravel really does feel warmer than the water.

We used 750 grams of Duplarit G laterite mixed with the bottom 1/3 of the gravel. With the laterite, Dupla supplies some "root starter" tablets that are also mixed with the gravel. We suspect these tablets are essentially houseplant fertilizer containing an initial dose of nitrates and phosphates.

Once the laterite/gravel substrate was in place, the rest of the gravel was layered on top. We made no provisions for terracing the substrate. We have found that aquarium gravel has a very strong tendency to become level over time. Any elevation changes will be accomplished with different sized plants.


Once the substrate was in place, we filled the tank with tap water at 80 degrees F. Our water is very soft, having a GH (general or calcium harness) of about 1.5 degrees (25 ppm) and a KH (carbonate hardness) of about 0.5 degrees (10 ppm). We like to keep our plant tanks at a GH of about 3 and a KH of about 4.5 to match the requirements of the plants, fish and pH buffering we desire.

Initially, we used appropriate amounts of Dupla KH and KH+GH tablets. These tablets are essentially sodium bicarbonate and calcium carbonate in a convenient to use form. We will probably create our own hardening "recipe" after our current supply of these tablets is exhausted.

When the KH+GH tablets are dissolved in water, they form a milky solution that is useful for determining water flow patterns. The solution was poured in the trickle filter sump and monitored as it was dispersed by the water currents.


Since the higher carbonate hardness in the tank raises the pH, we add CO2 to both lower the pH to the desired level and provide a carbon source for the plants. We add enough CO2 to maintain an initial pH of about 7.1. This is compatible with the needs of black mollies, which are used to break in the tank and provide algae control until the plants have established themselves.

The relationship of CO2, pH and KH is determined by the action of the carbonic acid generated when CO2 is added to water. At a fixed KH, adding or removing CO2 will lower or raise the pH accordingly. At a specific point in this relationship, the carbonate hardness provides a strong buffering capacity that prevents the pH from dropping as biological activity occurs.

This buffering is a result of the chemical properties of weak acids. When a weak acid dissociates in water, the ratio of the acid-base pairs formed has a logarithmic relationship. If you plot the acid-base ratio against pH, you would find that above or below a certain pH, the curve is very flat, i.e., as acids or bases are added to the solution, pH will change quite a bit. Around a certain pH, called the equilibrium point, the curve is nearly vertical, meaning that as acids or bases are added there is very little change in pH. Note that there can be more than one equilibrium point and they are different for every acid.

The relevant equilibrium point for carbonic acid is pH=6.37. This is ideal for planted aquaria since the desired pH is usually slightly above this point and the typical trend is for the pH to be lowered because of acids generated by biological processes. Since the starting pH is above the equilibrium point and any shift is toward it, a lot of acid can be "buffered" before the pH will fall below that point.

As another example, phosphate buffering is used in some commercial pH stabilizers. The important equilibrium point for dihydrogen orthophosphate is at pH=7.21. In this case, if the pH is at 7.0 and more acid is added, the pH will move away from the equilibrium point and will reduce the acid buffering capacity. Because of this, phosphates are a better buffer for an African cichlid tank, where the pH is usually higher. Also note that phosphates tend to produce algae, whereas carbonates are benefical to plants.


There are four components that make up a plant's requirements for growth. The first is light, which is well provided for by the metal halide lights. The second is nitrogen and phosphorus compounds, which are provided by the waste products of the fish (and the "root starter" tablets). The third is carbon, which is provided by the CO2. The last is the proper trace elements, which in our case are provided by Dupla products.

Plant growth will be decided by the "Liebig Minimum Law", which states that growth will be limited by that component which has the lowest concentration. That is, if you have great light, lots of nitrates and phosphates (i.e., lots of fish), plenty of CO2 but few trace elements, the lack of trace elements will hold back the plant growth.

We have tried many commerical fertilizers and have found the Dupla products to be the best for our conditions in terms of providing all the essential elements for fish and plant health. There are three components to the Dupla line. The first is Duplagan, which is used at water changes and adds organic acids beneficial to slime coat production on the fish. This product looks a little like water treated with peat and seems to contain essential vitamins and minerals.

The second is DuplaPlant tablets, which contain a large dose of iron and other trace elements and is also used when water is changed. DuplaPlant tablets supply those trace elements which are not toxic in large doses (enough to last between water changes) and which are stable enough to remain in the water until used by the plants. The predominant components are potassium, iron, sodium, manganese boron, calcium and nickel.

The third component is DuplaPlant 24 "Daily drops", which contain mostly iron and some trace elements that are either toxic in large doses or are unstable. As the name implies, this is added daily in very small quantities (5 drops per day for the SST).

The initial iron levels are higher than we normally maintain due to the laterite. After being set up, we measured 0.5 ppm of Fe in the water before adding and other fertilizers. This should be used up fairly rapidly by both the initial plant growth and by the nitrifying bacteria as they develop. After one week, we measured 0.35 ppm. In the long run, we will try to keep iron in the 0.1 to 0.15 ppm range.

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This page was last updated 23 June 1999