Algae is an ongoing part of any endeavour involving water. Most aquarists and pond-keepers don’t like it, and we get a lot of questions about how to control it.
Algaecides – chemicals which kill algae – are illegal in Canada. While this is a bit of a nuisance for people who want a sparkling clean pond or aquarium, we regard it as being generally a good thing. It’s too easy simply to kill off what you don’t want, without considering the underlying situation that nurtured the algae in the first place. Killing it off won’t fix that, and may make it worse.
Algae needs light and food to thrive. “Food” in this case is nutrient in the water, and “nutrient” usually means something is decaying. That something could be fish poop, plant material or uneaten food. Whatever it is, the algae is removing it from the water. This is a good thing, unsightly as the algae may be.
To kill algae without harming your other aquarium or pond denizens, remove either the light or the food. Simply turning off the aquarium light won’t do it, because ambient light, while not enough to keep aquarium plants happy, is plenty for algae. You need to cover the aquarium with something which will keep out all the light, while still permitting air to get in. Our usual suggestion is a black plastic garbage bag. Open it, drop it over the aquarium, and don’t seal the bottom. After a few days the algae will die from lack of light.
Of course, if you don’t deal with the nutrients, the algae will be back as soon as the lights come on again. Reducing the nutrients involves better filtration, regular water changes and probably a reduction in feeding.
To reduce light and nutrients in a pond, use plants. Surface plants like water lilies cut off the light. Oxygenators like hornwort use nutrient in the water, reducing its availability for algae. Of course, you should also make sure that your fish aren’t overfed, that your pond isn’t overcrowded, and that you practice good pond maintenance with water changes and adequate filtration.
Managing the factors that nourish algae is more work than simply killing it off with an algaecide, but in the end your pond, and your fish and other plants, will be healthier and happier for it.
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