Red algae in freshwater aquariums is usually caused by an abundance of nutrients such as nitrates, phosphates, and silicates. This can be due to overfeeding of fish or other sources like decaying organic matter that accumulates in the tank.

Red slime algae isn't actually algae but a bacteria known as cyanobacteria, an evolutionary link between bacteria and algae dating back at least 3.5 billion years ago. Growth of red slime algae is a common problem in saltwater aquariums, and it should be dealt with to ensure a healthy environment for your aquarium plants and animals.

Often called "Red Slime Algae" or "Cyano", Cyanobacteria, is actually not a type of algae at all. Rather a photosynthetic bacteria that are opportunistic in our aquariums. It exists in all aquariums and will only rear its nasty face when the conditions are right for it to grow prolifically.

Red algae are much less harmful to aquatic life and vegetation than brown algae. When grown in a contained environment, it can be beneficial to the fish tank. Unicellular red algae perform photosynthesis and add oxygen to the fish tank, making it a pleasant place for fish to live.