Aquarium plants can absorb ammonia and other nitrogen compounds. This improves the quality of aquarium water since these compounds are toxic to fish. However, in order for this process to work, you must keep plants under ideal conditions -- and aquarium plants are harder than they look.
The Nitrogen Cycle
The nitrogen cycle makes it possible to keep fish in aquariums for more than a few days. In the nitrogen cycle, fish produce ammonia -- a toxic byproduct of their metabolism. Various bacteria break this ammonia down into nitrite, then nitrate. Plants can assist in this process by absorbing these nitrogen products at each step of the cycle.
Plants and Nitrogen
Unlike animals, to plants ammonia is a food source, not a poison. Depending on the exact aquarium plant, they can absorb nitrogen compounds like ammonia and nitrite through their leaves and root systems. Plants use these as starting materials to build nitrogen compounds of their own for energy and structure. So under ideal situations, plants can be used as a "nitrogen sink," a place to dump these compounds and get them out of the aquarium water where they can harm fish.
The process of establishing the bacteria that run the nitrogen cycle is called cycling an aquarium. The original method of establishing these bacteria is to add a few hardy fish at a time to give the bacteria colonies time to expand to the nitrogen load. However, a variation on cycling, called silent cycling, takes advantage of plants' ability to absorb nitrogen and ammonia. In this process, fish are added as they would in traditional cycling. However, in silent cycling, you densely plant the aquarium. Plants absorb excess nitrogen. This speeds the cycling process and softens ammonia "spikes," which can be hard on the fish.
Caring For Plants
However, you can only reap the benefits of this process if your plants are thriving. Aquarium plants require good nutrition, lighting and carbon dioxide supplements. Only use fertilizers designed for aquariums. Aquarium plants usually need different ratios of nutrients -- partly since they have so much nitrogen available. Aquarium plants need powerful lighting, ranging from 1.5 to 5 watts per gallon of aquarium volume. Additionally, most plants benefit from the addition of carbon dioxide to their tank. You can add carbon dioxide with injector systems or liquid supplement.