How to Kill Off Plankton Without Harming Fish

Planktonic or free-floating algae ruin the look of an aquarium. These plankton float in the aquarium water, discoloring and clouding it. Aquarium hobbyists dub this problem "green water" or "pea soup water." It typically happens in new aquariums, and you can take several steps to clear it up.

  1. Lighting

    • Since planktonic algae depends on light to survive, you can control the problem by limiting light in the aquarium. If you set up the tank in direct sunlight, you can reduce the problem by blocking out some of the light with aquarium backgrounds or poster board. You can also reduce green water by turning off aquarium lights for several days. In the absence of light, green water plankton cannot survive.

    Live Aquarium Plants

    • Introducing live aquarium plants can prevent and reduce a plankton problem. Higher plants require the same nutrients as free-floating algae. And aquarium plants do a better job at absorbing and using these nutrients. This starves the algae and reduces green water. However, live plants are harder than they look. It takes some understanding of their physiological needs to keep them healthy. Neglected plants will actually decrease water quality, so try this approach only if you know how to keep aquarium plants happy.

    Biological Filtration

    • Biological filtration helps keep green water in check. As an aquarium matures, bacteria growing in the gravel eat fish wastes ammonia and nitrogen. They can absorb enough nutrients to starve out green water plankton just like live plants. You can encourage the growth of beneficial bacterial organisms by not overcrowding your tank, or by using certain aquarium filters designed to encourage their growth.

    Water Changes

    • Water changes are another means of helping the problem -- but they are a stopgap measure and don't necessarily fix the underlying problem. A 50 percent water change can help clear up the water. However, in some areas, tap water is rich in phosphates. Phosphates encourage blooms of green water plankton in the first place. You can find out if your local tap water has a high level of phosphates by asking a nearby pet shop or by purchasing an aquarium phosphate test. If your local water is high in this nutrient, try using water purified with reverse osmosis for filling your fish tanks. Some aquarium chemicals are marketed as being able to clear green water, but their effectiveness can be rather hit or miss.