Troubleshooting: Algae and slime
How do I get rid of cyanobacteria (red slime) in my reef tank?
Red, purple, burgundy or dark green sheets on the sand and rock, often with trapped bubbles. They peel off in a mat and come back where flow is weak.
Last checked . How we check
What should I check first?
- Where does it grow? Patches in the same quiet corners point at flow.
- Test nitrate and phosphate. Both too high and too low have been linked to it.
- Check your RO/DI with a TDS meter.
- Look at feeding: how much, how often, and whether frozen food is rinsed.
What is most likely causing it?
Most likely first. The order is our reading of how often the sources link each cause to this symptom.
- Dead spots with little flow, where detritus settles. The mats grow in the same low-flow corners and on sand behind rock.
- Nutrients out of balance. High nitrate and phosphate from heavy feeding, or one of them near zero while the other is not. BRS lists both directions as drivers.
- Poor source water. RO/DI reading above 0 TDS, or tap water used for top-off.
- Carbon dosing. Started after beginning vodka or another carbon source (keeper reports, typical, check).
What should I do?
- Siphon the mats out during water changes. It will come back at first; that is expected.
- Fix flow: add or turn a powerhead so the patches are moving water.
- Cut back feeding a little, rinse frozen food, and clean filter socks or floss more often.
- Replace RO/DI cartridges if the TDS reading is above 0.
- If nutrients are near zero, let them rise a little; if they are high, lower them slowly.
- Only then consider a treatment. BRS calls chemical removers a last resort because they can lower oxygen: run extra aeration and follow the label exactly.
What not to do
- Starting with a chemical remover before fixing flow and feeding: the mats tend to return.
- Big changes to nutrients in one go.
When should I get help?
Something dying right now? Go to the emergency playbooks.