Cyanobacteria (Red Slime): Causes and Treatment

Cyanobacteria isn't actually algae, despite the common "red slime algae" name. It's a photosynthetic bacteria, and that distinction matters because it responds to different pressure than true algae does. It shows up as a reddish or brownish film on sand and rock, and it can appear even in tanks testing near-zero nitrate, since phosphate alone is often enough to sustain it.

What actually causes it

Excess nutrients are the usual driver, but not always the whole story. Low or stagnant flow zones, like sand beds and rockwork dead spots, are common starting points because detritus accumulates there undisturbed. Cyanobacteria die-off also consumes real dissolved oxygen and raises CO2, which is why outbreaks are often linked to nighttime pH dips and low-oxygen stress, not just nutrient levels on their own.

The treatment ladder, safest first

1. Manual removal. Siphon visible mats out directly rather than brushing them, which just spreads viable fragments. In one Reef2Reef discussion, a hobbyist resolved an outbreak in six days using siphoning alone, with no chemical treatment at all.
2. Increase flow. Eliminating dead spots removes the stagnant conditions cyanobacteria favors, without touching your tank's biology.
3. GFO or biopellets. GFO (granular ferric oxide) targets phosphate specifically. Biopellets work differently, feeding bacteria that consume nitrate and some phosphate through carbon dosing. That bacterial process needs real time to colonize - Brightwell Aquatics' own guidance says to allow at least 4 weeks before expecting a measurable change in nutrient levels. Add them in 25% increments to avoid crashing nutrients too fast, standard practice for any new biopellet reactor.
4. UV sterilizer, with a caveat. UV only affects free-floating organisms passing through the water column. It has no direct effect on cyanobacteria already established on rock or sand, though it may reduce spore load somewhat.
5. Chemical treatment, last resort. Products like Chemiclean (erythromycin-based) kill cyanobacteria on a fixed 48-hour protocol. The manufacturer states it's safe for fish, corals, and nitrifying bacteria when dosed exactly per label with heavy aeration. That claim doesn't fully hold up in practice. Long-running Reef2Reef threads including one 8+ page thread and a separate warning thread document tanks crashing after chemical treatment, generally tied to skipping the aeration step or the die-off's oxygen demand outpacing the tank's dissolved oxygen supply. If you use it, follow the label dose exactly, add aeration beyond the stated minimum, and don't skip the water change afterward.

Mistakes that make this take longer

Treating symptoms without addressing the underlying nutrient or flow problem is the most common mistake. Cyanobacteria that gets removed but not starved comes back within days. The opposite mistake, driving phosphate and nitrate all the way to undetectable, is also a known trigger for dinoflagellate outbreaks, which are considerably harder to deal with. Aim for low but detectable nutrients, not zero, a principle covered in more depth in our green hair algae guide.

The bottom line

Start with manual removal and flow correction, since both carry zero biological risk and resolve real cases on their own. Layer in GFO or biopellets for the underlying nutrient load. Save chemical treatment for outbreaks that don't respond to the safer steps, and when you do use it, follow the label and add extra aeration rather than trusting the manufacturer's safety claim alone.