Cyanobacteria is usually a sign of excess nutrients and poor water flow, not a disease you treat with medication

Cyanobacteria — the slimy red, brown, or blue-green film that spreads across rocks and substrate — thrives when nitrate and phosphate levels stay high and water movement is weak. It is not a true bacteria and does not respond to antibiotics. Removing it means cutting off what feeds it: excess nutrients, stagnant spots, and decaying matter. The fastest way is a combination of water changes, better circulation, and manual removal, not a chemical fix.

Cyanobacteria blooms in established tanks more often than new ones because detritus builds up over months. Fish waste, uneaten food, and dead algae break down into nitrate and phosphate — the fuel cyanobacteria needs. If you see it spreading, your tank is telling you that something is not being removed fast enough.

Key Takeaways

  • Cyanobacteria grows in response to high nitrate and phosphate levels, so the first step is a 25 to 50 percent water change and a thorough substrate cleaning.
  • Increase water flow with an additional powerhead or by repositioning existing ones so that no area of the tank sits still for more than a few seconds.
  • Remove the cyanobacteria manually by siphoning it out or scrubbing it off rocks, then dispose of it outside the tank so it does not break down inside.
  • Reduce feeding and increase the frequency of water changes until nitrate stays below 20 ppm and phosphate stays below 0.5 ppm.
  • If cyanobacteria returns within weeks, the underlying cause — usually a clogged filter, dead spots in the tank, or overfeeding — has not been fixed.

Do a large water change and clean the substrate thoroughly

Start by removing 25 to 50 percent of the water. This dilutes the nitrate and phosphate that cyanobacteria feeds on. While the water level is down, use a gravel vacuum or siphon to clean the substrate, paying special attention to areas under rocks and in corners where detritus collects. Cyanobacteria often sits on top of a layer of decaying matter — removing both at once stops it from regrowing in the same spot.

When you refill, use saltwater that has been mixed to the same salinity as your tank — usually 1.023 to 1.025 specific gravity. If you use water that is too fresh or too salty, you will stress the fish and corals without solving the nutrient problem. Replace the water over 15 to 20 minutes so the temperature and pH do not swing.

Increase water circulation to eliminate dead zones

Cyanobacteria thrives in corners and under overhangs where water barely moves. Add a second powerhead or reposition your existing one so that water flows across every part of the tank at least once every few seconds. The goal is no area of still water where detritus can settle and decay.

If you already have two powerheads, check that they are not both pushing water in the same direction. Aim them so their flow patterns cross or circle the tank. A common mistake is placing both powerheads on the same side — this creates a dead zone on the opposite side. You can test flow by dropping a small piece of food in different spots and watching how fast it moves. If it sinks straight down, that area needs more flow.

Manually remove the cyanobacteria

While the water change and flow improvements work, remove the visible cyanobacteria by hand. Use a siphon to vacuum it directly off the substrate, or scrub it off rocks with a soft brush or old toothbrush. The key is to remove it completely — if you leave pieces behind, they will regrow quickly.

Do not let the cyanobacteria fall back into the tank. Siphon it into a bucket and throw it away, or scrub it into a net and dispose of it outside the aquarium. If you leave it in the tank, it will break down and add more nutrients to the water, defeating the purpose of removing it.

Test and reduce nitrate and phosphate

After the water change, test your nitrate and phosphate levels. Nitrate should be below 20 ppm; phosphate should be below 0.5 ppm. If nitrate is still above 40 ppm or phosphate is above 1 ppm, the source of excess nutrients is still active — usually overfeeding, a clogged filter, or dead spots you missed.

Use a phosphate test kit and a nitrate test kit (both are inexpensive and available at aquarium stores). If levels are still high after cleaning, reduce feeding by half and do another 25 percent water change in three days. Repeat testing every few days until the numbers drop. This takes patience — it can take two to three weeks for nutrient levels to stabilize if the problem has been building for months.

Check your filter and feeding routine

A clogged filter does not remove waste efficiently, so detritus builds up and feeds cyanobacteria. Check your mechanical filter media — the foam or floss that catches particles — and rinse it in old tank water (not tap water, which kills the beneficial bacteria). If the filter is clogged, water bypasses it and waste stays in the tank.

Overfeeding is the most common cause of persistent cyanobacteria. Fish should eat all food within two to three minutes. If food sinks to the bottom uneaten, you are feeding too much. Cut the amount by a third and watch for three days. If cyanobacteria stops spreading, you have found the problem. Smaller, more frequent feedings (two or three times a day instead of one large feeding) also help because the fish eat more of what you give them.

When to consider stronger interventions

If cyanobacteria returns within two weeks despite water changes and better flow, the underlying cause has not been fixed. Before trying chemical treatments or UV sterilizers, check these common problems: Is the substrate still getting cleaned weekly? Are you still overfeeding? Is the filter getting rinsed regularly? Are there dead spots where water does not flow?

Some aquarists use UV sterilizers or ozone generators to kill cyanobacteria, but these only work if you have already fixed the nutrient problem. A UV sterilizer kills the visible cyanobacteria but does not remove the nitrate and phosphate that caused it, so the bloom returns. Chemical treatments like antibiotics are rarely effective and can harm beneficial bacteria in the tank. The real fix is always the same: lower nutrients, better flow, and less detritus.

Frequently Asked Questions

Is cyanobacteria dangerous to fish and corals?

Cyanobacteria itself is not toxic to most fish, but it smothers corals and rocks, blocking light and water flow. Some species of cyanobacteria produce toxins, though this is rare in home aquariums. The bigger problem is that it signals high nutrient levels, which also cause unwanted algae and poor water quality.

Can I use hydrogen peroxide to kill cyanobacteria?

Some aquarists dose hydrogen peroxide directly into the tank, but it is risky. It can kill cyanobacteria but also damages beneficial bacteria and can stress fish if the dose is too high. Manual removal and nutrient control are safer and more reliable.

Why does cyanobacteria come back after I remove it?

It returns because the nutrients that feed it are still there. If you clean the tank but do not reduce nitrate and phosphate, or if you do not improve water flow, cyanobacteria will regrow within days or weeks. Fix the cause, not just the symptom.

How often should I do water changes to prevent cyanobacteria?

In a healthy tank, 20 to 25 percent weekly is standard. If you have had cyanobacteria, increase to 25 to 50 percent twice a week until nitrate and phosphate stay low for a month. Then return to weekly changes, but keep testing — if levels creep up again, cyanobacteria will follow.

Should I replace my substrate if cyanobacteria is bad?

Replacing substrate is rarely necessary. Thorough vacuuming removes most detritus. Only replace it if the cyanobacteria is so thick that you cannot clean it out, or if the substrate itself is breaking down and creating excess nutrients.