At first you may be thinking, “The Good? Really? How can there be anything good about algae?” Well, the truth is algae are not all bad. Algae play several important roles in your pond or aquarium.
Algae influence carbon and nutrient cycling. They are in aquatic systems what plants are to terrestrial systems. They are at the bottom of the food chain using light as an energy source allowing them to convert the gas carbon dioxide into cellular tissue. And just like plants, algae generate oxygen. They increase dissolved oxygen in the water which good for higher organisms like fish. Much of the earth’s oxygen (>70%) comes from the oceans and algae are responsible for a significant portion of this. Algae, like plants, reduce carbon dioxide, a major greenhouse gas. So give those lowly algae some credit like your life depends on them, because it does.
Let’s focus more on how algae affect a smaller system like a pond or aquarium. Algae get their nutrients from whatever is dissolved in the water. Probably most important to ponders and aquarists is their role in nitrogen cycling. Nitrogen compounds like ammonia and nitrite are very toxic to fish. In water, nitrogen exists in various chemical forms and they are converted by chemical and biochemical reactions. Nitrogen cycling is very complex and involves a lot of chemical and biochemical processes, but for the sake of this article we will keep it very simple.
Nitrogen is a critical element for forming living tissue, proteins, amino acids, etc. Only oxygen, carbon, and hydrogen are more abundant in cellular tissue. Most often, the major source of nitrogen entering the water of a pond or aquarium comes from fish food. Other sources are dead vegetation (grass, leaves, etc.) and perhaps animal wastes like bird droppings. Fish food is converted by fish into energy and cellular tissue. Excess nitrogen in the food is excreted in feces as ammonia. Food that is not eaten decomposes and releases its nutrients into the water creating more ammonia. Bacteria quickly convert this toxic ammonia to nitrite. Unfortunately, nitrite is even MORE TOXIC than ammonia. But, luckily for the fish, other bacteria quickly convert nitrite to nitrate. Nitrate is not very toxic and is the preferred nitrogen source for plants, algae, and bacteria. So there you have the nitrogen cycle for aqueous systems: Living organism→ ammonia→nitrite→nitrate→living organism.
Algae are a very important presence in your pond or aquarium. They, along with other microorganisms like bacteria, help minimize the levels of toxic forms of nitrogen in the water. They produce oxygen. They help stabilize your pond/aquarium. Where algae and bacteria differ is that bacteria can convert the nitrogen to various gases that escape the system. Algae, when they die will release the nitrogen back into the system to repeat the nitrogen cycle. This is why most ponds and aquarium have some sort of biofilter; a place where conditions are altered to maximize bacteria’s response to excess organic matter and nitrogen.
So algae are important in a healthy aquatic system but too much algae is a different story. Measures to limit algae include: Reduce the amount of nitrogen entering the system (fish food, plant litter, animal wastes, etc.); mechanically remove the algae and other organic matter (particulate filter, raking, etc.); or improve the bacterial response to excess nitrogen (increase or improve bacterial activity).