When you keep fish, sooner or later you will have to deal with algae. So anyone with goldfish should know how to address different kinds of alga, whether you keep your goldfish in an aquarium or in a pond. Fortunately, managing algae is not impossible.
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Alga is not always a bad thing. For example, goldfish need some green algae in their diet and may nibble at algae in their habitat. Feeding them algae flakes every now and then is good for them. Spirulina is a blue-green alga that is found both in goldfish food and often is an ingredient in health food products for humans.
Types of Alga
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There are several common types of alga you're likely to encounter. Green algae tend to occur in stringy strands (filaments) or as free-floating cells. When it is free-floating, green alga looks like green water. Blue-green algae are a bluish-green slime and are more of a concern than green algae, as they can be poisonous to fish and very few things eat it. Brown algae occur in sheets on aquarium surfaces.
Treating Aquarium Algae
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In the goldfish aquarium, physically scrubbing algae off surfaces is probably the best way to address algae in the short term. For a longer-term solution, keep up on aquarium maintenance. Regular water changes remove fish waste and other nutrients that algae need. Also, changing your lighting may help. If you have blue-green algae, try keeping the tank in darkness for a week. If you have brown algae, try increasing the illumination. Algae-eating critters can help with green algae, but don't expect them to do all the work.
Treating Pond Algae
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One of the best ways to control algae in a goldfish pond is to add hardy pond plants. If your plants are thriving, they will do a better job of capturing nutrients than algae, out-competing it for resources. Adding shade to your pond, perhaps by planting a tree next to it, will limit the amount of light reaching the algae, making it harder for them to grow. You should consider algae-killing chemicals only as a last resort, as such products will decrease the water quality substantially.