Red algae thrives in established tanks—remove it by cutting light, improving water flow, and doing manual cleanup

Red algae (also called coralline algae when it's the purple-pink kind you want, or red slime algae when it's the slimy pest you don't) spreads fastest in tanks with low water movement, excess nutrients, or lighting that runs too long each day. You stop it by making the tank less hospitable: reduce light hours from 10–12 down to 8, increase water circulation with a powerhead or wavemaker, do a 25 percent water change weekly, and manually remove visible growth with a scraper or siphon. Most tanks see improvement within two to three weeks once you change these conditions.

The difference between the two matters because coralline (the desirable purple-pink crust on rocks) and red slime (the slimy red-brown film) need different responses. Coralline is actually a sign of a mature, stable tank and you want to keep it. Red slime is a nuisance that indicates an imbalance. This guide covers red slime removal; if you have the purple coralline and want to encourage it, the steps are reversed—add more light and calcium.

Key Takeaways

  • Red slime algae grows fastest when aquarium lights run 10 or more hours daily; cutting to 8 hours or less starves it without harming most corals or fish.
  • Poor water circulation traps nutrients that feed algae, so adding or repositioning a powerhead to create flow across all surfaces is one of the fastest fixes.
  • Manual removal with a scraper or by siphoning the slime away works when ready but only lasts if you also fix the underlying conditions.
  • Weekly 25 percent water changes reduce the nutrient load that sustains algae blooms and should be part of your routine regardless.
  • Chemical treatments and UV sterilizers work but are last resorts; they do not fix the root cause and the algae often returns within weeks.

Reduce your lighting schedule to 8 hours or less per day

Red slime algae needs light to photosynthesize, and most tanks run lights for 10–12 hours to mimic a natural day. Cutting that to 8 hours starves the algae without harming fish or most corals. If you have light-demanding corals (like SPS or LPS), you can hold at 8–9 hours instead of going lower. The key is consistency—set a timer on your light fixture so it turns on and off at the same time each day.

Start by dropping from your current schedule to 8 hours and hold it there for two weeks. If the algae is still spreading, drop to 7 hours. Most tanks show visible improvement by week three. You may notice your corals look slightly less vibrant at first because they are receiving less light, but they adapt quickly. Fish do not care about light duration as long as they have a consistent day-night cycle.

If you have a programmable LED fixture, you can also dim the lights to 50–75 percent intensity rather than cutting hours. This works because algae is opportunistic—it grows fastest under bright, long-duration light. Dimming reduces the energy available to it while still supporting coral growth.

Increase water circulation with a powerhead or wavemaker

Red slime thrives in dead spots where water barely moves. Stagnant areas trap decaying food, fish waste, and other nutrients that feed algae. A powerhead (a small submersible pump) or wavemaker (a powerhead on a timer that pulses) breaks up these dead zones and keeps nutrients suspended so your filter can remove them.

Position the powerhead to create flow across the substrate and rock surfaces where algae grows. Aim for water movement that is visible but not so strong that it blows your corals around. If you already have a powerhead, try repositioning it—sometimes a different angle eliminates the dead spot that was feeding the algae. A tank 40 gallons or smaller usually needs one powerhead; larger tanks may need two positioned on opposite sides.

Run the powerhead 24 hours a day. If you use a wavemaker, set it to pulse every 10–15 seconds so it mimics natural wave action. Better circulation also improves oxygen levels and helps your biological filter work more efficiently, so this change benefits the whole tank.

Manually remove visible red slime with a scraper or siphon

While you are fixing the underlying conditions, remove the algae you can see. Use an algae scraper (a plastic or magnetic tool with a blade) to scrape it off rocks and the glass. For the substrate, use a siphon or turkey baster to suck the slime away without disturbing your sand or gravel too much. Do this twice a week while the bloom is active.

Manual removal does not kill the problem—it just buys time while your other changes take effect. The algae will grow back within days if you do not also reduce light and improve flow. But removing visible growth prevents it from shading corals and makes the tank look better while you wait for the conditions to shift.

Be gentle around corals and delicate invertebrates. If the algae is growing on a coral, use a soft brush or your fingers to gently rub it away rather than scraping hard. Some corals can handle a light scrape; others will retract or bleach if you are rough.

Do a 25 percent water change every week

Regular water changes remove excess nutrients (nitrogen and phosphorus) that fuel algae growth. A 25 percent change weekly is standard maintenance, but during an active algae bloom, do it consistently without skipping. This means removing one-quarter of your tank water and replacing it with fresh saltwater mixed to the same salinity as your tank.

Mix your saltwater at least a few hours before the change so the salt dissolves completely and the temperature matches your tank. Use a siphon to remove water from the substrate, which also pulls out detritus (dead organic matter) that would otherwise break down and feed algae. After you remove the old water, slowly pour in the new saltwater so you do not disturb your substrate or corals.

If your tank has been running without regular changes, you may have a nutrient buildup. In that case, do a 25 percent change weekly for a month, then drop back to your normal schedule (usually every two weeks). This accelerates the removal of the excess nutrients that are sustaining the bloom.

Test your water parameters and adjust if needed

Red slime algae thrives when phosphate or nitrate levels are high. Test your water with a phosphate test kit and a nitrate test kit (both available at aquarium stores). Phosphate should be below 0.1 ppm; nitrate should be below 20 ppm. If either is higher, the algae has a food source that you need to address.

High phosphate usually comes from tap water, fish food, or decaying organic matter. If your tap water is high in phosphate, use a phosphate-removing resin in your filter or do smaller, more frequent water changes with bottled saltwater. High nitrate usually means you are overfeeding or not doing water changes often enough. Cut feeding by 25 percent and increase water change frequency to every 5–7 days until nitrate drops.

You do not need to bring these numbers to zero—some algae growth is normal in any tank. The goal is to bring them low enough that the algae cannot bloom. Once you have reduced light, improved flow, and brought nutrients down, the algae will slow dramatically.

Consider chemical treatments or UV only as a last resort

Products like Red Slime Remover (usually containing hydrogen peroxide or other algaecides) and UV sterilizers kill algae quickly, but they do not fix why the algae was there in the first place. After two to four weeks, the algae often returns because the tank conditions have not changed. Chemical treatments can also harm sensitive corals or invertebrates if you overdose, so read the label carefully and dose for your exact tank volume.

A UV sterilizer is a device that runs water through a chamber with ultraviolet light, killing algae spores and bacteria. It works, but it is expensive (usually $100–$300) and requires a separate pump. It is worth considering only if you have tried the basic fixes (light reduction, flow, water changes) for four weeks with no improvement.

If you do use a chemical treatment, continue the other fixes at the same time. Stop the treatment as soon as the algae is gone, because running it indefinitely can harm your tank's biological balance. Most experienced aquarists avoid these products and rely on the environmental fixes instead, which are cheaper and more reliable long-term.

Frequently Asked Questions

Will red slime algae kill my fish or corals?

Red slime itself is not toxic, but a heavy bloom can shade corals and reduce oxygen levels if it covers a large area. Fish are not harmed directly. The real problem is that the algae indicates an imbalance in your tank—excess nutrients or poor flow—that can stress your livestock over time. Remove it to keep your tank healthy.

How long does it take to get rid of red slime?

Most tanks see visible improvement within two to three weeks of reducing light and improving flow. Complete removal usually takes four to six weeks. If you see no change after four weeks, check your phosphate and nitrate levels—you may have a nutrient problem that needs a different approach.

Can I use bleach or vinegar to clean algae off rocks?

You can soak rocks in a 10 percent bleach solution (one part bleach to nine parts water) for 24 hours, then rinse thoroughly with fresh water before returning them to the tank. Vinegar does not kill algae effectively. Only do this if the rock is not colonized by beneficial bacteria or corals. For rocks in an active tank, scraping by hand is safer.

Is purple coralline algae the same as red slime?

No. Purple or pink coralline algae is a hard crust that most aquarists want to keep—it is a sign of a mature, stable tank. Red slime is a soft, slimy film that spreads quickly. If you have coralline and want more of it, increase light to 10–12 hours and add calcium supplements. The fixes for red slime are the opposite.

What if the algae comes back after I fix it?

It usually means one of the conditions has drifted back. Check that your light timer is still set to 8 hours, your powerhead is still running, and you are still doing weekly water changes. If all three are consistent and the algae returns, test your phosphate and nitrate—something in your routine may have changed.