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Cyanobacteria, Dinoflagellates, Hydrogen Peroxide, Reefkeeping

Hydrogen Peroxide, Cyano & Dinos: A Practical Reefkeeping Guide

AI-generated illustrations of dinoflagellates on the left and cyano on the right in reef aquariums under blue lighting.

Mitchell Ballou |

Peroxide can help knock back the visible growth, but it doesn’t fix the water chemistry or other conditions that helped it take hold. We work on those underlying conditions from the start, because the slime can come back after treatment.

What dinos and cyano can look like

Dinos can form brown strings or dusty patches; cyano often forms cohesive mats. These images show possible appearances, but looks alone don’t confirm identification. Our dino identification guide can help narrow it down.

AI-generated split banner showing a reef tank with brown dinoflagellate growth on the left and a tank with burgundy cyanobacterial mats on the right, both under blue lighting.
Dinoflagellates on the left; cyano on the right.
AI-generated illustration of brown dinoflagellate patches and bubble-bearing strands across sand and bare reef rock under blue lighting.
Dinoflagellates: one example of brown growth with mucus strands and bubbles. Some outbreaks look more like a dusty coating.
AI-generated illustration of cohesive burgundy cyanobacterial mats across a blue-lit reef aquarium sandbed with bare rock.
Cyano: an example of cohesive burgundy mats with uneven edges and small trapped bubbles.

Our peroxide treatment in seven steps

At New Dawn Aquaculture, we occasionally use hydrogen peroxide for nuisance slime outbreaks in our tanks. We’ve found it tends to help alongside attention to the tank conditions behind the growth. Here’s our routine for a quick reference or reminder; results vary between tanks and organisms.

  1. Stop the return pumps.

    Isolate the display from the sump. Keep wavemakers running for circulation and surface movement, and maintain any circulation or aeration the isolated sump needs.

  2. Calculate the actual display water volume.

    Allow for rock and sand displacement at the treatment water level. Exclude the isolated sump. Use the water actually being treated, rather than the tank’s advertised capacity.

  3. Loosen the slime with a turkey baster.

    Gently lift accessible growth into the water column, including growth around corals. Use a gentle stream so you don’t injure coral tissue.

  4. Dose 1 mL per 10 US gallons.

    Use ordinary store-bought 3% hydrogen peroxide, checking the label. Measure the dose and add it to moving display water, avoiding concentrated contact with coral tissue. For example, 40 US gallons takes 4 mL. For litres, divide the actual display volume by 37.85 to get the dose in mL.

  5. Wait 30–45 minutes, then restart the return.

    Check that the return pump and filtration restart normally. This is our waiting interval, not confirmation that all peroxide has broken down.

  6. Add beneficial bacteria.

    After restarting the return, we add Palustris Live Bacteria - Bacto Fuel, following its current dosing directions. Our aim is to help beneficial bacteria recolonize cleared surfaces; this does not guarantee recolonization or elimination of the nuisance growth.

  7. Repeat daily for about two weeks.

    Check livestock before every dose and track the growth. If it persists after that period, revisit identification and tank conditions before increasing the dose or continuing treatment.

If fish or corals show distress, stop peroxide dosing. Rapid breathing, loss of balance, or unusual coral retraction or tissue changes call for reassessment. Maintain circulation and aeration, and check water quality.

We also review nitrate and phosphate trends, debris buildup, and flow. Clearing the slime doesn’t necessarily fix what caused it. If you need help, send us photos, recent test results, and a little background on your tank.

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