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Balancing Lights, CO2 and Fertilization

by Karen A. Randall <>

I believe that the biggest cause of frustration and failure with live plants comes from people not understanding that the balance of light and nutrients and CO2 must always be maintained. In the aquarium, CO2 is usually the limiting factor. If you increase either the amount of light, or the available nnutrients without also increasing the CO2, algae will get the upper hand.

The book The Optimum Aquarium by Horst and Kipper covers thoroughly the high tech, high cost method of maintaining this balance in a planted tank. Follow their instructions to the letter, and I am sure you will be successful with your plants. Your wallet will also be much thinner.

For those who find that this sort of system is out of reach, or just too technically oriented, there are other ways of maintaining beautiful, healthy aquarium plants.


Proper lighting is crucial to the success of a planted tank. Unfortunately, as with many other facets of plant care, there are almost as many theories as there are people growing plants. There are two points where almost everyone is in agreement. First, tropical plants require a tropical photoperiod for optimal growth. Between twelve and fourteen hours seems to be best. Second, the single strip light that comes as part of a standard aquarium "set up" is not adequate for good plant growth.

I use at least two full length strip lights over each planted tank, although it is possible to maintain a few very low light plant species with less. Depending on other parameters, particularly if you are using a CO2 system, you will probably need more light than this.

Some people advocate the use of incandescent bulbs in addition to the florescent, claiming that they enhance the growth of some plants, particularly Echinodorus sp.

There are a number of full spectrum florescent bulbs on the market these days varying in price from expensive to out of sight. The ones I use are Vita Lites. I believe they are the least expensive of the group. The "Power Twist" bulbs are a little brighter, but they are more expensive. You can get more light out of what ever bulb you use by using a good reflector.

I have settled on a combination of Vita Lites and Philips Daylight bulbs. The daylight bulbs are about one third the cost of the Vita Lites, and seem to do an adequate job.

Whatever florescent lights you decide on, the bulbs should be changed long before they wear out. The amount of light produced by these bulbs drops off with use, and by the time they are a year old, they are no longer adequate for use on a planted tank. I alternate, replacing one bulb every six months to maintain as even lighting as possible.

There are other more expensive light sources available, such as those manufactured for reef tanks. They are being used with great success by some people, particularly those who are using CO2.

The most important thing to remember is that it is hard to supply too much light to your plants. The limiting factor will be how much light you can supply and still keep the balance between your nutrients and CO2 supply.

Carbon Dioxide

CO2 supplementation is being used by a number of people these days with great success. There are some plants that do phenomenally well with supplemental CO2 that are difficult to keep without it. This is true particularly in the case of fast growing plants with high light requirements. If you can afford it, and enjoy playing with equipment, CO2 supplementation is certainly worthwhile. However, a CO2 system is not a requirement for a beautifully planted tank.

CO2 is present in every aquarium. It is given off by the fish, and as a by product of nitrification. Again, the important thing to remember is that light, CO2 and nutrients must always be in balance. Otherwise, you will not obtain optimum plant growth, and you risk unwanted algae problems.

In my low-tech tanks, my plants do well enough using the CO2 available from sources within the system. I am careful not to drive off CO2 unnecessarily by making sure that my filter returns are below surface level to produce as little surface turbulence as possible. Depending on the balance of plants and fish in your tank, you may find that you need to provide additional water movement when the lights are off.


Another important consideration for a planted tank is fertilization. Many people are under the impression that the only food needed by aquarium plants is "fish poop". While there is a certain element of truth to this belief, there is more to the story.

I venture to say that all modern aquarists are aware of the need for regular partial water changes for the health of their fish. This is also true for your plants. While plants do need nitrogen, preferably in the form of ammonium, as a food source, the amount of nitrate available in the average tank is many times greater than the amount that would be found in the water where the plants were collected. Elevated nitrate levels will cause severe algae problems long before the fish show any adverse effects. This is also true of phosphate.

Although there is more than enough nitrogen and phosphate available for your plants in one form or another, there are other substances which may need to be supplemented.

Aquatic plants obtain nutrients in two different ways. Almost all are able to absorb some nutrients through their leaves, directly from the water. Many also obtain nutrients from the substrate through their root system.

For this reason is important that the proper nutrients be available both in the substrate and dissolved in the water.

If you do some reading, you will find that there are almost as many methods of fertilizing plants as there are experts. There are even a number of articles available that have in depth analyses of the chemical composition of fish waste according to the diet of the fish. While these are interesting exercises, the fact remains that due to differences in tap water, lighting, plant species, fish load, and feeding and maintenance practices, every tank will need slightly different supplementation.

I try to use a pragmatic approach to the problem. The methods I use have not been worked out in a scientific manner, but they work with my tap water in my tanks. I'll walk you through my thought process to help you decide how best to supplement your own tanks.

Since I know that there is more than enough nitrogen available in various forms, there is no need to supplement. I use only fertilizers meant for aquariums, and I stick to those that include little or no nitrogen. This completely rules out fertilizers formulated for house plants since one of their major components is nitrogen. The same holds true for phosphate. Water changes help keep the nitrate levels low in the tank as long as the tap water itself is low in these substances. In some areas, the tap water can contain excessive amounts of nitrate due to agricultural run-off. In other places, municipalities are adding phosphate to tap water to decrease lead leaching from old pipes. In both of these instances, the aquarist may need to resort to a purification system of one sort or another to remove these substances from the water.

Most tropical plants require a great deal of iron. Even if your water source has high concentrations of iron, it is unlikely that it will remain in a form usable by the plants for more than a few hours after it is added to the tank. Unless you have a very low tech tank with slow growing plants and low lighting, you will probably need to supplement with iron. I currently supply iron to my plants in several ways. I use laterite in the substrate of all tanks. I also use plant food tablets that are high in iron. They are pushed into the gravel around the roots of the plants every eight weeks.

In a low tech set up, this, plus water changes and feeding your fish may very well supply enough iron for the needs of your plants.

There are many other trace elements needed by your plants. In a tank with brighter lighting, supplemental CO2 and faster plant growth, you will probably find that you need a balanced trace element supplement. I am not a chemist, or a botanist, so I depend heavily on the expertise of those who market aquarium plant foods. The plant tablets that I use as iron supplementation provide other trace elements as well. In my high tech tanks I also use a daily liquid trace element supplement.

Finally, I change 25-30% of the water in my tanks on a weekly basis. I am convinced that if I had the time to change this much water twice a week, my plants would benefit from it. In addition to keeping the nitrate level as low as possible, it replaces whatever trace elements are available in my tap water.

As mentioned earlier, every tank is an entity unto itself. Slavish adherence to a particular method, be it mine or that of someone else, is likely to result in disappointment. Part of the fun of working with live plants is the process of thinking through your problems, and fine tuning your system to maximize the beauty of your plants.

Play with various levels of light, different types of fertilizer, and a CO2 system if you are so inclined. Just keep the need for balance of these elements in your mind at all times. If you experience an algae problem, you need to cut back on light or fertilizer, or increase the amount of CO2 available to the plants, or the number of fast growing, nutrient consuming plants. If you see an increase in plant vigor, you are probably on the right track.

This article was originally printed in The Daphnian, publication of the Boston Aquarium Society as monthly instalments of my column, ``Sunken Gardens''. BTW, the column was voted ``best column'' in the North East Council of Aquarium Societies in both 1993 and 1994.
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This page was last updated 29 October 1998