1. How Fish Waste Turns into Ammonia and Nitrate in the Water
Fish waste and leftover food release ammonia, which contains nitrogen. Bacteria in the water, known as nitrifying bacteria, change this ammonia into nitrate through a process called nitrification, passing through nitrite along the way [1]. In closed water systems like aquariums, the nitrogen that is released is thought to eventually remain in the form of nitrate. This article does not discuss how much nitrate accumulates in aquarium water.
2. Why Water Plants Prefer Ammonium as Their Nitrogen
Aquatic plants, such as water plants, absorb ammonium from the water and change it into organic nitrogen, which becomes part of their bodies [2]. The amount plants absorb can greatly influence the concentration of ammonia-nitrogen in the water [2]. A 2022 paper on plants growing underwater calls ammonium the "preferred nitrogen source" [3]. This does not mean they cannot absorb nitrate.
3. Why Too Much Ammonium Can Harm Water Plants
In an 18-month pond experiment, two types of water plants, *Elodea nuttallii* and *Vallisneria americana*, were given high amounts of ammonium. At an ammonium-nitrogen level of 7 mg/L, both plants maintained high coverage (the percentage of the pond surface covered) and height for a short time. *Vallisneria americana* was evaluated as being able to withstand up to 18 mg/L of ammonium-nitrogen [3]. These numbers are from pond experiments. They are not safe values for home aquariums. They do not directly apply to plants living with fish in a home tank. Ammonia is also toxic to fish. The amount water plants can tolerate and the amount fish can survive must be considered separately. SAFETY NOTE: These are values from pond studies, not safety standards for home aquariums. Ammonia-nitrogen is toxic to fish. The tolerance of water plants and the safety of the whole tank are separate issues.
4. How Fast Rootless Hornwort Absorbs Nutrients
*Ceratophyllum demersum* is a submerged plant with no roots; it absorbs nutrients from the water through its leaves and stems. In an experiment, it quickly absorbed nitrogen and phosphorus from the water during the first 4 days, lowering the concentration of inorganic nitrogen and phosphorus [4]. In the water with reduced nutrients, the level fell below what was needed for *Lemna minor* in the same container to grow [4]. However, the competitor was *Lemna minor*, not algae that attaches to surfaces in an aquarium. No materials were found in this survey that directly show water plants and algae competing for nutrients.
5. Did Chemicals Help Hornwort Suppress Duckweed?
The main mechanisms concluded for *Ceratophyllum demersum* suppressing *Lemna minor* were the rise in water pH and the lack of nutrients like nitrogen [4]. When nutrients were added to water containing *Ceratophyllum demersum*, the growth of *Lemna minor* did not change much. Therefore, substances that stop other organisms (allelopathy) were not considered the main cause [4]. In a pond with *Potamogeton crispus*, water from the pond suppressed the growth of harmful blue-green algae (*Microcystis*). This effect continued while the water plants were present. It is thought that substances suppressing the growth of other organisms were released [5]. Thus, the way water plants suppress other plants or algae differs by species and location. The findings here are about *Ceratophyllum demersum* versus *Lemna minor* and blue-green algae in ponds. They cannot be applied to all water plants and algae.
6. How Dead Water Plants Return Ammonia to the Water
Plant absorption greatly lowers ammonia concentration in the water. But when plants die and decompose, ammonia is released back into the water [2]. From this, removing dying leaves early seems to be a way to prevent nutrients from returning to the water. However, no materials in this survey directly measured this effect in an aquarium. This point remains a guess based on the materials. Water plants help keep water clean, but if left alone, they do not continuously protect water quality.
7. How to Investigate Water Plants in an Aquarium Yourself
In homes, schools, or shops with aquariums, observe water plant leaves for one week at the same time each day. Do not put hands in the water or shake the tank; just look. Write down changes in leaf color, the position of dying leaves, and what sticks to the leaf surface. This records changes in the plants and allows comparison of daily conditions. If you find dead leaves, tell the person caring for the tank. If you want to know more, you can find books about water plants and how to grow them in libraries. Use words like "water plants," "water quality," and "nitrification" as clues.