Red Slime Algae (Cyanobacteria) in a Reef Tank: Causes & Fixes

Red slime algae in a saltwater tank is cyanobacteria, a photosynthetic bacterium rather than a true alga. It forms red, maroon or purple slimy mats on sand and rock, usually because of low water flow, a nitrogen-to-phosphate imbalance, overfeeding or ageing lights. Fix the cause, siphon it out, and it will fade within weeks.

This guide is for reef and fish-only keepers dealing with red slime now. It covers how to tell cyanobacteria from dinoflagellates and red macroalgae, what causes it, a step-by-step removal plan, chemical treatments, the blackout method and, honestly, what eats red slime in a saltwater tank.

Updated October 2026 by the team at Natural Seawater Supplier, a Croydon, London-based supplier of live natural seawater for marine aquariums, delivering across the UK 7 days a week.

Key takeaways

• Red slime algae is cyanobacteria (blue-green “algae”), a bacterium that photosynthesises; it comes in red, purple, green, brown and black forms.
• The most common triggers are dead spots with low flow, excess dissolved organics from overfeeding, an imbalance where phosphate is present but nitrate is very low, and old or wrong-spectrum lighting.
• Cyano peels off in sheets and is easy to siphon; dinoflagellates are stringy, snotty and bubble-laden; red coralline and red macroalgae are firmly attached.
• Very few animals eat cyanobacteria reliably; Cerith and Trochus snails and some conchs graze a little, but no clean-up crew will fix an outbreak.
• Treatments such as Boyd Chemiclean can clear mats in about 48 hours, but they drop oxygen and the slime returns unless flow and nutrients are fixed.

What is the red algae in my saltwater tank?

If it is a slimy, red-to-purple film on the sand or rock that lifts off in sheets, smells slightly earthy and often traps small gas bubbles, it is almost certainly cyanobacteria. Despite the name “red slime algae”, it is a bacterium. Cyanobacteria are among the oldest life forms on Earth and produced much of the oxygen in our atmosphere, which is a good reminder of how tough and adaptable they are. The University of California Museum of Paleontology has a clear introduction to cyanobacteria if you want the science.

Cyanobacteria can fix nitrogen from the air, which is why they can thrive when your nitrate test reads zero. They also produce sticky slime that smothers corals and blocks light, so a large outbreak is more than cosmetic.

Cyano vs dinoflagellates vs red macroalgae vs coralline: how to tell

Organism	Appearance	Texture	Where	Key test
Cyanobacteria (red slime)	Deep red, maroon, purple; also green or black	Flat slimy sheets, peels off easily	Sand, low-flow rock, glass edges	Lifts in a sheet with a turkey baster; does not regroup after shaking
Dinoflagellates	Brown, golden, sometimes green	Snotty strands with lots of bubbles	Sand, rock, coral bases	Shake a sample, strain it: cells regroup under light within an hour
Red macroalgae (e.g. Gracilaria, Botryocladia)	Red or burgundy, leafy or bushy	Firm plant-like structure	Attached to rock	Has a holdfast and does not peel off
Coralline algae	Pink, purple, red crust	Hard, rock-like	Rock, glass, pumps	Cannot be wiped off; needs scraping
Diatoms	Brown or golden dust	Powdery film	Sand and glass in new tanks	Wipes away easily, not slimy

Telling cyano from dinoflagellates matters because the fixes are almost opposite. Cyano is usually linked to excess organics; dinos tend to appear when nitrate and phosphate are near zero. If you are unsure, Bulk Reef Supply has a good guide to controlling dinoflagellates, and our saltwater algae identification guide covers every common type.

What causes red algae in a saltwater tank?

Low or poorly directed flow

Red slime almost always starts in dead spots: behind rockwork, in sand corners and in the lee of powerheads. Organic waste settles there and cyanobacteria colonise it. If red algae on the sand forms in the same place every time, flow is your first suspect.

Nitrogen-to-phosphate imbalance

Because cyanobacteria fix their own nitrogen, they win when phosphate is available but nitrate is very low. Tanks that have been “stripped” with heavy nitrate-reducing media or carbon dosing often grow red slime. Keep nitrate around 2–10 ppm and phosphate around 0.02–0.08 ppm in a mixed reef.

Overfeeding and dissolved organics

Uneaten food, phosphate-rich frozen food juice and dirty filter socks raise dissolved organic carbon, the fuel cyano loves. Feed what your fish eat in about two minutes; our guide to how much to feed marine fish gives amounts per fish size.

Old bulbs or the wrong spectrum

Cyanobacteria absorb strongly in the red and orange part of the spectrum. Ageing T5 or metal halide lamps drift towards that end, which is why red slime often appears 9–12 months after a tube change. Replace T5s every 9–12 months, avoid running extra red channels on LEDs, and keep the photoperiod to around 8–10 hours. Our reef tank lighting schedule explains how.

New tanks and tap water

Cyano is common during the first few months as part of the normal algae succession. Tap water used for top-off or mixing salt adds phosphate and nitrate that make it worse.

How to get rid of red slime algae: step by step

1. Confirm it is cyano. Use the table above. If it is dinoflagellates, do not follow the nutrient-stripping advice below.
2. Test. Record nitrate and phosphate. Remember that cyano absorbs nutrients, so readings can look artificially low during an outbreak.
3. Fix the flow. Add or redirect a powerhead so the affected area has visible movement without blasting the sand. Most reefs need roughly 20–50 times tank volume per hour.
4. Siphon it out. During a 15–20% water change, use airline tubing or a turkey baster to lift the mats from sand and rock. Stir the top few millimetres of sand to remove trapped detritus. Do this every few days.
5. Reduce organics. Cut feeding by a third, rinse frozen food, change filter socks every 3–4 days and run fresh activated carbon.
6. Improve export. Make sure the protein skimmer is producing dark, dry skimmate, and grow chaeto in a refugium; see our guide to nutrient export.
7. Check the lights. Replace old tubes and trim the photoperiod. A blackout (below) can speed things up.
8. Be patient. Most outbreaks fade in two to four weeks once the cause is fixed. If not, consider a treatment.

The blackout method

A blackout starves cyanobacteria of light. Siphon out as much as you can, do a water change, then switch off the lights and cover the tank with a towel or bin bag for three days (72 hours). Keep pumps, heaters and the skimmer running, add an airstone and feed lightly. Most corals tolerate three days of darkness well. Afterwards, bring the lights back gradually over a few days. A blackout works best combined with nutrient and flow fixes; on its own, the slime often returns.

Chemical treatments for cyanobacteria

Boyd Chemiclean

As of 2026, Boyd Chemiclean is still widely sold as a cyanobacteria remover in 2 g and 6 g powder and liquid forms. The 2 g pack treats up to about 1,135 litres (300 US gallons). The manufacturer markets it as safe for fish, corals, invertebrates and beneficial bacteria and it is not sold as an antibiotic, although the formula is not published. Typical instructions are:

• Turn off the protein skimmer, UV and ozone, and remove carbon before dosing.
• Add strong aeration with an airstone, because treatment can lower oxygen levels.
• Leave for 48 hours, then do a water change of at least 20–25% and turn the skimmer back on slowly (it may overflow).
• Run fresh carbon afterwards and repeat only if the label allows.

Treat the label as the authority and dose for the true water volume, not the tank’s nominal size. Watch fish for heavy breathing.

Antibiotics such as erythromycin

Older articles recommend erythromycin phosphate. In the UK, antibiotics for fish are prescription-only veterinary medicines, they can damage your biological filter and they encourage resistance. We do not recommend them for nuisance cyanobacteria.

Other products

Bacterial “competitor” additives and carbon dosing can help some tanks by out-competing cyanobacteria, but results vary and overdosing carbon sources can itself trigger slime. Stick to one change at a time.

What eats red slime algae and cyanobacteria?

Very little, and nothing reliably. Cyanobacteria produce compounds that make them unpalatable to most grazers. Animals that will eat some red slime include:

Animal	How much cyano it eats	Notes
Cerith snails	Some, on sand and glass	Good sand sifters; also eat diatoms and detritus
Trochus snails	Some films, mostly other algae	Can right themselves; reef safe
Fighting and tiger conchs	Moderate on sand	Need a sand bed and enough film to graze
Nassarius snails	Mainly detritus, not cyano	Help by removing waste that feeds it
Small reef-safe hermit crabs	Occasional	Pick at mats; see our reef-safe hermit crab guide

Our reef-safe algae eaters guide explains which grazers suit which algae. A clean-up crew helps prevent mats re-forming once the cause is fixed, but it is not a cure.

How to stop red slime coming back

• Keep powerheads clean and aimed to remove dead spots.
• Do 10–20% water changes every one to two weeks with clean, low-phosphate water; see how often to change water in a saltwater aquarium.
• Do not strip nitrate to zero; keep it low and stable.
• Replace T5 tubes on schedule and avoid long photoperiods.
• Use RODI water for top-off.

The water you use for changes matters. Our natural seawater typically tests at nitrate 0–0.9 ppm and phosphate 0–0.05 ppm and arrives ready to use, with salinity of 34–36 ppt. For a regular supply, get a delivery quote or read our complete guide to getting rid of algae in a saltwater tank.

Frequently asked questions

What is the red algae in my saltwater tank?

It is most likely cyanobacteria, also called red slime algae. It is a photosynthetic bacterium that forms red or purple slimy sheets on sand and rock, often with trapped bubbles, and it peels off easily when siphoned.

What causes red algae in a saltwater tank?

Red slime is usually caused by low water flow, excess dissolved organics from overfeeding, phosphate being available while nitrate is very low, and old or red-shifted lighting. New tanks and tap water top-off also make outbreaks more likely.

What eats red algae in a saltwater tank?

Very little eats cyanobacteria reliably. Cerith snails, Trochus snails, conchs and some small hermit crabs graze small amounts, but they will not clear an outbreak. Fixing flow and nutrients is what removes it.

How do I get rid of red algae on the sand?

Siphon the mats and the top few millimetres of sand during water changes every few days, add flow to the affected area and reduce feeding. If it persists after two to four weeks, a three-day blackout or a cyanobacteria remover such as Chemiclean can help.

Is red slime algae harmful to fish and corals?

Red slime is not directly toxic to most fish, but thick mats smother corals, block light and can lower oxygen at night. Some cyanobacteria release irritating compounds, so large outbreaks should be removed promptly.

Is Chemiclean safe for a reef tank?

Chemiclean is marketed as safe for fish, corals and invertebrates, but it can lower oxygen. Turn off the skimmer, add an airstone, follow the dose for your true water volume and do a 20 to 25 percent water change after 48 hours.

Bottom line

Red slime algae in a saltwater tank is cyanobacteria, and it is telling you something about flow, organics or lighting. Confirm it is not dinoflagellates, siphon it out, fix dead spots, feed less and keep nitrate and phosphate low but not zero. Use a blackout or Chemiclean only to speed things up once the cause is under control.

Sources

External sources cited in this article (checked October 2026):

• Introduction to cyanobacteria (UC Museum of Paleontology, Berkeley)
• Guide to controlling dinoflagellates (Bulk Reef Supply)