red slime

bad water quality.

You have too much nutrients in the water, which can be from using tap, over feeding, over stocking, or your bulbs might be going bad.

What are the results of your water test (actual numbers please). Also how big is your tank, what do you have in it, how long has it been set up, filtration and other equipment (including powerheads)
 
Its pretty common in semi-new tanks that are still going through phases of maturing.

But little_fish is right, once your water quality clears up and your lights are in the right spectrum, it should go away.
 
also, oxygen exchange. not enough oxygen exchange results in high levels of carbon dioxide in the water, which is a potential carbon source for algae. but the causes fish mentioned are more prevalent.

In addition, there's lore that higher alk is better for preventing cyano, as is a moderately high pH (say, 11-12 dKH and 8.4 pH)
 
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