[Sk8r]

Understand, without cyanobacteria, Earth would be a dead planet.
It gave us oxygen after the Permian Extinction, before there were dinosaurs.
It travels in the air. It's the ancestor of the chloroplasts (photosynthesizing element) in every green plant on Earth.
So don't curse it.

But when it decides to help out in your tank, you'll find a red or brown film (depends on your lights) staining your sand---and growing to a half-inch thick blanket over everything, distinguished by little bubbles of oxygen. THAT's a problem.

Solving the problem is easy: and this process imitates a natural process on the reef, when storm rolls over an area. Draw the room curtains and turn out your tank lights for 3 days. If using metal halide, bring up actinic on the 4th day, regular lighting on the 5th. And WHILE your lights are out, SKIM. A good skimmer really helps. Be sure it's 'tuned' to work its best. This process will not hurt corals or fish! You may have to do this once monthly for several months running---like about 3 months. But it WILL get it. Do not do it more than once a month. And yes, your fish will appreciate being fed on schedule: they'll sleep mostly, but will wake up to eat.

Cyano is midway between a plant and an algae: it has characteristics of both: some kinds even crawl...a bit. It won't exit the tank at night and do in the poodle. It also is amazingly adaptable: you'll hear advice about turkey basters (actually a good idea if it's very, very thick: the more you can suck out, the better: but suck, don't blow!) And about nitrates and phosphates and all sorts of things---but that's actually not true, when it comes to ridding your tank of cyano, since all it REALLY has to have is light, water, and carbon. Carbon is an element present in everything alive, so there's no way you're going to rob it of nutrients just by tossing out nitrate and phosphate, desirable as that is to do. All we can do that it doesn't like is to turn out the lights.

Will it come back? Just about inevitably. It loves odd-spectrum light, so slanty sunlight reaching your tank from a window or bulbs in your light-kit that are expired or expiring are both situations it loves. Check your bulbs, see where the sun-track from that side window is for the season, but if it does come back, just do the lights-out thing and you'll see it less and less often.