What cyanobacteria is and why it spreads

Cyanobacteria is not algae — it is a photosynthetic bacterium that forms slimy, often blue-green or reddish mats on tank surfaces, substrate, and decorations. It spreads fast because it thrives in warm water with excess nutrients, low water flow, and poor lighting schedules. Unlike true algae, it can produce toxins that harm fish and invertebrates, making removal a priority rather than a cosmetic choice.

Cyanobacteria blooms usually signal one or more underlying problems: uneaten food decaying on the bottom, dead plant matter, infrequent water changes, or a filter that is not moving water effectively. The bacteria itself does not cause these conditions — it exploits them. Killing the visible bloom without fixing what caused it will bring it back within days or weeks.

Key Takeaways

  • Cyanobacteria thrives in warm, nutrient-rich water with poor flow, so the first step is removing decaying food and plant matter by hand and increasing water circulation.
  • A partial water change of 25 to 50 percent, combined with thorough gravel vacuuming, removes excess nutrients the bacteria feeds on.
  • Reducing light to 6 to 8 hours per day and repositioning lights away from affected areas slows growth while you address the root cause.
  • Antibiotics like erythromycin or tetracycline kill cyanobacteria directly but work best after you have removed decaying matter and improved water conditions.
  • If the bloom returns, check your filter intake for clogs, test water temperature (keep it below 78°F), and confirm your feeding amount matches what fish consume in 2 to 3 minutes.

Remove visible cyanobacteria and decaying matter by hand

Start by siphoning out as much visible cyanobacteria as you can reach. Use a gravel vacuum on the substrate where the bacteria often settles thickest, and wipe affected decorations and tank walls with a clean cloth or algae scraper. Do not worry about removing every trace — the goal is to reduce the bacterial load and the organic matter it feeds on.

While you are cleaning, look for uneaten food trapped in plants or under decorations, dead leaves on live plants, and any fish waste buildup. Remove these by hand or with tweezers. Dead plant matter is a major nutrient source for cyanobacteria, so trim any yellowing or decaying leaves and remove them from the tank entirely. If you have plants that are mostly dead, consider replacing them rather than leaving them to rot.

Increase water flow and reduce light exposure

Cyanobacteria grows slowly in water that moves constantly. Check that your filter outlet is not blocked and that water is circulating throughout the tank, especially in corners and behind plants where flow often stalls. If your filter is undersized for your tank volume, consider adding a small powerhead or adjusting the filter output to push water into dead zones.

Reduce your light schedule to 6 to 8 hours per day — cyanobacteria photosynthesizes, so less light means slower growth. If you have adjustable lighting, dim it to 50 percent intensity during the outbreak. Once the bloom is gone, you can return to your normal schedule. If the affected area is in a corner, reposition your light to avoid shining directly on that spot while you treat the problem.

Perform a large water change and vacuum thoroughly

Remove 25 to 50 percent of your tank water and replace it with fresh, dechlorinated water at the same temperature. This dilutes excess nutrients (nitrogen and phosphorus) that cyanobacteria feeds on. While the water level is lowered, vacuum the gravel aggressively, paying special attention to areas where the bacteria is thickest and where detritus collects.

After refilling, check your filter. If it is clogged or running slowly, clean the media in old tank water (not tap water, which kills beneficial bacteria). A filter that is not moving water at full capacity allows organic matter to settle and decay, which feeds the bloom. Repeat this water change and vacuum every 3 to 4 days until the cyanobacteria stops returning.

Use antibiotics if the bloom persists after cleanup

If cyanobacteria returns within a week despite improved water conditions and reduced light, consider an antibiotic treatment. Erythromycin and tetracycline are the most effective options and are sold as aquarium treatments at most fish stores. Follow the dosage on the package exactly — overdosing can harm fish and invertebrates, and underdosing will not kill the bacteria.

Antibiotics work best after you have already removed decaying matter and improved flow, because they kill the bacteria but do not address the nutrient problem that caused the bloom. Treat for the full duration listed on the package (usually 5 to 7 days), then perform a 25 percent water change to remove dead bacterial matter. Do not repeat the treatment more than once without waiting at least two weeks and confirming that conditions have improved.

If you have plants in the tank, note that some antibiotics can slow plant growth temporarily. If you have invertebrates like shrimp or snails, check the package label — some treatments are not safe for them. In that case, remove invertebrates to a separate container during treatment if possible.

Check temperature and feeding to prevent recurrence

Cyanobacteria thrives above 78°F, so if your tank runs warm, lower the temperature if your fish species allows it. Most tropical fish do well at 74 to 76°F, and lowering the temperature by even a few degrees slows bacterial growth significantly. If you do not have a heater, move the tank away from direct sunlight or warm air vents.

Overfeeding is one of the most common causes of cyanobacteria blooms. Feed only what your fish consume in 2 to 3 minutes, once or twice per day. Uneaten food sinks and decays, creating the nutrient-rich environment the bacteria needs. If you feed frozen or live food, thaw it first and remove any that sinks uneaten after a few minutes.

Monitor water parameters and adjust maintenance

If cyanobacteria keeps returning even after treatment, test your water for nitrate and phosphate levels. High nitrate (above 40 ppm) or phosphate (above 2 ppm) means nutrients are still accumulating faster than you are removing them. Increase the frequency of water changes to once per week, vacuum more thoroughly, and reduce feeding further.

Check your filter intake as well — if it is partially blocked by algae or debris, water is not circulating properly and organic matter settles on the bottom. Clean the intake tube and any pre-filter sponge weekly during an outbreak. Once the bloom is gone and you have gone two weeks without seeing new growth, return to your normal maintenance schedule but keep water changes at least weekly until you are confident the problem will not return.

Frequently Asked Questions

Is cyanobacteria dangerous to fish?

Some cyanobacteria species produce toxins that can harm or kill fish, especially if the bloom is thick and covers a large area. Even non-toxic strains reduce oxygen in the water as they decay. Removing it quickly is safer than waiting to see if your particular strain is toxic.

Can I use bleach or hydrogen peroxide to kill cyanobacteria?

Bleach will kill the bacteria but also harms fish and beneficial bacteria in the filter. Hydrogen peroxide at very low doses (a few drops per gallon) is sometimes used but is risky and unpredictable. Antibiotics designed for aquariums are safer and more reliable.

Why does cyanobacteria come back after I treat it?

Treatment kills the visible bloom but does not fix the conditions that caused it. If you still have excess nutrients, poor water flow, or too much light, the bacteria will regrow. Focus on removing decaying matter, improving circulation, and reducing feeding before relying on antibiotics alone.

Can I use UV sterilizers or ozone to get rid of cyanobacteria?

UV sterilizers kill some cyanobacteria cells as water passes through, but they do not remove the slimy mat or address the nutrient problem. They work best as a preventive tool after you have already cleaned the tank and fixed the root cause. Ozone is effective but requires special equipment and is risky for home aquariums.

How long does it take to get rid of cyanobacteria completely?

If you clean thoroughly, improve water conditions, and treat with antibiotics, visible cyanobacteria usually disappears in 7 to 14 days. However, it can return within weeks if you do not maintain the improved conditions. Plan on 4 to 6 weeks of consistent maintenance before you can be confident it will not come back.