Blue-green algae are technically not algae at all. They are bacteria. They were considered algae because they are aquatic and make their own food.
This group of bacteria is called the Cyanobacteria.
Look at the picture on the left. This aquarium is covered in Blue Green Algae. They are small and single-celled but grow in large enough colonies that you can see them.
They can have colors ranging form black to red to purple.
Like a lot of bacteria, these type of algae in your substrate are beneficial to the health of your saltwater aquarium. They feast on detritus (organic wastes).
They form a dark brownish-red gelatinous mat called red slime if you have poor water quality, high nutrients level and poor water circulation.
The picture on the right is the same aquarium a couple weeks later. Phosphate remover was added to the filtration system and water changes took place.
They form on rocks, gravel, and plants in your saltwater aquarium. They are also capable of producing toxins that poison the saltwater aquarium fish. They must be physically removed with a siphon during a partial water change.
There are several products on the market nowadays to help eliminate slime algae, but the key is to stay on top of your water chemistry, water changes, and water motion or circulation.
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Last update on 2018-07-20 / Affiliate links / Images from Amazon Product Advertising API