Cyanobacteria (red slime)

Identification

Slimy, sheet-like mats that peel or blow off in pieces with a turkey baster. Most often red to maroon or dark purple, and also green, black, blue-green or rust colored. Mats trap small bubbles during the light period, feel slick rather than gritty, favor low-flow corners, sandbeds and rock bases, and can return within a day of siphoning while conditions suit them.

Often confused with

Dinoflagellates are stringier and snottier with bubbles on strands and a strong day-night swing. Diatoms are a thin dusty brown film with no slime sheet. Purple coralline algae is hard, encrusting and cannot be blown off, while cyano lifts easily. Cyano mats also smear when rubbed, where filamentous green algae pulls away like wet hair.

What it targets

Sandbed, rockwork, frag plugs and the bases of corals. Thick mats shade coral tissue, trap detritus under the sheet, and irritate nearby invertebrates.

How it spreads

Nearly impossible to exclude: cells arrive on rock, frags, sand and in water changes. Blooms are fueled by detritus buildup, low flow dead spots, and nutrient imbalance, commonly phosphate present with nitrate at or near zero, so the mat faces little competition.

Control (safest first)

Siphon mats out regularly, ideally daily during a bloom, and blow detritus off rock before water changes so the food source leaves the tank. Raise flow in dead spots and remove the detritus traps where mats start: corners, under rock edges and still sand areas respond first. Rebalance nutrients rather than zeroing them: bring a bottomed-out nitrate up to a measurable level (keepers often target around 5 ppm) while keeping phosphate controlled with skimming, water changes and phosphate media if needed. Stripping both nutrients to zero can trade cyano for dinoflagellates. Support competing microbes: bacterial dosing and a maturing microbiome are reported to shrink cyano territory over weeks, paired with the flow and nutrient work above. Short light reduction (3-5 days) is used by some keepers as a reset alongside siphoning. Corals tolerate short reductions better than long blackouts, and mats return if the cause stays. ChemiClean and similar cyano products: widely reported to clear red cyano within about 48 hours when the label is followed, with strong aeration required during treatment and a water change after. Keepers also warn the skimmer can overflow heavily afterward, and that cyano returns when the nutrient and flow cause stays unfixed. Some keepers avoid antibiotic-type red slime removers over concern for beneficial bacteria; treat chemical options as the last step, not the first.

Natural predators

• Trochus snails→
• Cerith snails→
• Some hermit crabs

Field notes

A keeper with a 4-year-old tank, nitrate at 0 and phosphate at 0.05 ppm, was advised to raise nitrate toward 5 ppm, vacuum daily and increase flow, with a report that this pattern usually clears cyano in about a week without chemicals. ChemiClean users on Reef2Reef describe it clearing cyano reliably when the water stays well oxygenated, with the caution that the skimmer goes wild during and after treatment and a thorough vacuuming plus a big water change should bracket the dose. Other keepers push back on chemical-first habits in the same forums, arguing that a little cyano is part of normal biodiversity and that chasing sterile water is part of how tanks end up with dinoflagellates, so they fix flow and nutrients first.