Red slime algae. What is it? What causes it? Anyone who’s ever had a saltwater aquarium has had to battle this pest. We call it an algae, but scientists currently classify it as cyanobacteria. The deep red slime that covers rock, gravel and everything else is actually a photosynthetic bacterium. In nature, however, cyanobacteria play a key role in a reef’s ecology. Under healthy reef conditions these bacteria form microscopic mats that are part of the reef’s food chain. Cyanobacteria are grazed on by reef organisms and play an important part in the food chain. But, no one wants to see it growing in their reef aquarium, so how do you prevent red slime?

What does cyanobacteria look like?

Red Slime

Ordinarily you won’t see cyanobacteria when things are in balance, but with the right conditions, it can form into thick mat-like colonies in green, purple, black and red forms. These bacteria colonies look like sheets of slime on rock, sand and gravel. It can also grow in a refugium and on top of macroalgae.

What causes red slime to grow?

Since the bacteria are already in your aquarium, all it takes is something to “trigger” the microbes to reproduce a slimy colony. Cyanobacteria are photosynthetic.  They use light energy and nutrients to grow. Microbiologists have found that cyanobacteria can obtain nitrogen from nitrogen gas. This gas comes from the decay of organic debris in the rock, sand and gravel. As the debris decomposes, nitrogen gas is released and captured by the slime. Under normal circumstances this nutrient recycling is one of many “microscopic” processes that keep the oceans and our reef aquarium healthy and balanced. Cyanobacteria also need phosphate, which is normally in very low quantities on the reef. The bacteria have the special ability to release a compound that dissolves organic matter, releasing the phosphate. The bacteria absorb the phosphate, fueling the growth process. Are you making the connection between organic sludge build-up and stimulation of red slime algae?

Does poor lighting cause red slime?

Before LED and metal halide reef lighting, most reef hobbyists used banks high-output fluorescent tubes. They started to notice that if the light bulbs were not changed on a regular basis red slime sometimes appeared. There seemed to be a loose connection between aging fluorescent bulbs and cyanobacteria. In some cases, the tank was also dirty and neglected with lots of organics, which triggers red slime. Some thought the connection between lighting and slime algae was just a myth. But more recently microbiologists discovered that cyanobacteria can thrive in poor lighting and outcompete other organisms. Scientists found that under low light cyanobacteria can fuel growth by absorbing light in the far-red spectrum. So, while other organisms in the ecosystem are inhibited, red slime can march forward and take over. But there’s more. There’s also a theory that blue light, like the kind produced by LED reef lighting, can inhibit cyanobacteria’s photosynthesis process. All this appears to confirm that under the right circumstances, red slime has the advantage in a poorly-lit reef aquarium.

How to prevent and control red slime

Red slime prevention tips diagram

The first approach is to keep your reef clean. Siphon out visible detritus on and behind the live rock. If you’re using sand or gravel, give it light siphoning now and then. Even though you can’t see a lot of sludge, there’s plenty on the microscopic level. That’s where it all starts, with invisible cyanobacteria and invisible food particles. If you allow sludge build-up, you’re asking for a red slime problem.

Good water movement in the tank keeps the tiny particulate matter suspended so it can be removed by the mechanical filter, like a filter sock. The water also flushes the live rock, pushing organic particles out of the crevices.

If you’re using filter socks, clean them often. Don’t like cleaning them? A fleece filter will do the job for you.

Keep your protein skimmer working efficiently. A skimmer can strip out fine organic particulate matter. Every little bit helps so make sure to service your skimmer regularly.

If you’re a fan of T-5 lighting, change out the bulbs on a regular basis. The bulbs decline in output and PAR as time goes on. Aging bulbs also require more energy to start and run.

What about chemical treatments?

Red Slime treatments
Chemiclean, Dr Tim's and Blue Life red slime treatments

A number of red slime treatment products are available to help knock down a bad slime problem. The active ingredients are considered trade secrets but are usually either an erythromycin (antibacterial) compound or a sludge-digesting bacteria. No matter which product you use, follow the directions carefully and don’t over-dose.


Sooner or later you’re going to see some red slime in your reef tank. Cyanobacteria are a normal and healthy component of reef’s ecosystem. You’ll never eliminate it from your tank. The idea is to keep it in check by knowing how to prevent red slime by limiting its food source removing sludge and detritus. Proper feeding, water changes and using RO water all increase the overall health and stability of the tank and make it difficult for red slime to take over. If you’re using T-5s, change them out periodically to maintain output and proper PAR.