Blue-Green Algae in Fish Tanks

Blue-Green Algae in Fish Tanks

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Blue-Green algae in fish tanks is a common and treatable problem. It can form in your tank at any time and isn’t really harmful to your fish. It can, however, be dangerous to any live plants you have.  It’s also just not very pretty to look at. Understanding what blue-green algae is, how it forms, and how to treat it can help you keep your fish tank clean and beautiful.

What is Blue-Green Algae?

Blue-green algae in your fish tank is actually a cyanobacteria. Cyan means blue, so this is literally a blue bacteria. It grows similarly to plants, using photosynthesis. Often it is blue-green, but the color of it can vary to shades of brown, black or even red. It looks like slime forming in your tank and it can cover anything. If it covers the leaves of your plants, keeping them from getting enough light, it can kill them.

blue-green algae in fish tanks


Blue-green algae has a very distinctive smell. Some describe it as earthy, musty, or swampy. Knowing the smell means you can actually detect blue-green algae very early on and save yourself some hassle in getting rid of it.

What causes blue-green algae to form?

Blue-green algae in fish tanks can grow from several causes.  First, a slow current in your fish tank can cause algae to grow. Moving water keeps it from getting stagnant. You’ll notice this in ponds and lakes as well. Those that have a clear source of water entering them and a current flowing through them have cleaner water than those with a very slow current.

Next, debris in your substrate can cause algae. Sure, there is always going to be a little bit of debris, but if you don’t have cleaner fish, or if you’ve been overfeeding, extra debris will end up there. With nothing to remove it, that debris can grow stagnant and cause this cyanobacteria to form.

Finally, too much light in one area of the aquarium can cause the blue-green algae to form as well. If there’s an area of your tank, specifically a corner, that is getting almost constant light, this can aid in the growth of cyanobacteria.


How do I get rid of blue-green algae?

First, try removing as much of it as you can manually. You can use a scraper, a siphon, a toothbrush, whatever works.  Clean the tank. Next, try changing the water more often and cut back on feeding, if overfeeding has been a problem. You’ll lessen the amount of debris in your substrate this way.

Clean your filter and increase your water flow a little bit. Just a little more current can help to cut down on algae formations. Finally, rearrange your plants and decorations if you can, to make sure no area is getting exposed to constant light.

If the algae really seems to have taken over, there are chemicals and medicines you can use to get rid of it. Give us a call and we’ll walk you through some stronger methods of dealing with blue-green algae in fish tanks if you need it!