Plants derive their own food energy from light. Chlorophyll is made of photoreceptive molecules that are able to absorb photons from light and use that energy to convert carbon dioxide and water into glucose and oxygen. At the center of this type of molecule is a single magnesium atom, surrounded by four nitrogen atoms. Nitrogen is vital to plant life. Fish waste contains nitrogen in the form of ammonia. Aquatic plants are able to use that nitrogen, though usually not directly.
Sources of Ammonia
Chemically, the ammonia molecule is a nitrogen atom surrounded by three hydrogen atoms. It readily dissolves in water, but in air it is a gas. Ammonia is abundant in nature because it is present in animal waste and decomposition.
Aquatic Plants and Ammonia
Because ammonia dissolves in water, aquatic plants have an advantage over their terrestrial counterparts in that the molecules do not dissipate out of reach of their roots. For that reason, aquatic plants can readily take in ammonia and use the nitrogen for making more chlorophyll. Aquatic plants that have access to copious amounts of nitrogen tend to be lush and green, with much foliage. Nitrogen is also used in every plant cell as part of its DNA and proteins, so it is vital to cell division and reproduction. However, too much nitrogen can harm aquatic plants, making them more temperature-sensitive, more attractive to pests and less able to reproduce because of so much foliar growth.
Deprived of adequate amounts of nitrogen, plants take on a yellow cast as the green chlorophyll is destroyed. The plant slows in growth, produces fewer leaves and eventually will die.
Ammonia and Nitrate
Even though aquatic plants are capable of using ammonia directly, that's not actually how they normally intake nitrogen. More commonly, aquatic plants take in nitrate, which chemically is a nitrogen atom bonded with three oxygen atoms, instead of the hydrogen atoms that make ammonia. Ammonia in water generally converts to nitrate before aquatic plants access it.
The Nitrogen Cycle
Plants and animals essentially swap nitrogen back and forth through a process known as the nitrogen cycle. In simplified terms, animals eat plants, then produce waste which plants consume in return.
Ammonia-Converting Bacteria and Aquatic Plants
With the nitrogen cycle, plants and animals interact symbiotically, but an important part of this relationship is the work of bacteria. Specifically, when fish and decay release ammonia in water, first one group of bacteria known as nitrosomas convert the ammonia to nitrite (NO2). At that point, another group of nitrobacter bacteria convert the nitrite to nitrate (NO3), which is the most usable form for plants.