Red Algae Fish Tank Complete Guide: Cyanobacteria Treatment

Red algae in freshwater is a confusing umbrella term: some hobbyists mean reddish black beard algae, others mean the slimy red-purple cyanobacteria sheets that smell earthy, and a few genuinely encounter freshwater red rhodophytes. This red algae fish tank complete guide from Gensou Aquascaping, 5 Everton Park separates the three, then walks through the step-by-step Singapore-appropriate treatment for each. Local PUB tap water sits soft and slightly acidic with very low KH, which favours cyanobacteria blooms when nitrate drops to zero and organic waste accumulates in dead-flow corners — a combination we see weekly in HDB planted tanks running low-tech setups.

Identify What You Actually Have

Rub a sample between two fingers. Slimy, sheet-like, peels off in one piece with a distinctive earthy or swampy smell? That is cyanobacteria, also called blue-green algae despite often being red, purple or black in freshwater. Tough tufts that refuse to pull off, dark red to near-black, on leaf edges and hardscape? That is red variant black beard algae (BBA). Wispy pink-red filaments more like very fine hair in low-light setups? True freshwater red algae, rare but possible.

Cyanobacteria Is Not Actually Algae

Cyanobacteria is a photosynthetic bacterium, which is why antibacterial treatments work and standard algicides do not. It thrives where nitrate is undetectable, flow is poor, and organic waste accumulates — classic symptoms of an under-fed planted tank. If your test kit reads 0 ppm nitrate, cyanobacteria is nearly inevitable in Singapore’s 28-30°C tanks where bacterial metabolism runs fast. The counterintuitive first step is to raise nitrate to 10-15 ppm, not lower it.

Three-Day Blackout Method

Cyanobacteria is light-dependent and crashes quickly without it. Before starting, siphon out every visible mat, do a 50% water change, clean the filter sponges in tank water, and dose KNO3 to reach 10 ppm nitrate. Then cover the tank completely with black bin bags or a thick towel for 72 hours, turn off the lights, and leave the filter and CO2 running. Feed fish sparingly once during the blackout. On day four, remove the covering, do another 50% water change, and you should see roughly 80-90% reduction.

Erythromycin Treatment: The Nuclear Option

If blackout fails or you have heavy infestation across plants you cannot afford to stress, erythromycin at 200 mg per 40 L for five days clears cyanobacteria reliably. Maracyn from Seachem and API E.M. Erythromycin are sold at select Singapore retailers and Shopee for SGD 18-32 per course. Remove activated carbon first, expect a temporary bacterial wobble in the biofilter, and run fresh carbon for a week after the treatment to strip residuals. Test ammonia daily during and after — a mini-cycle is possible.

Red BBA: Treat Like Regular BBA

If your red algae is actually the reddish variant of black beard algae, cyanobacteria tactics do not apply. Spot-treat hardscape and affected leaves with Seachem Flourish Excel at 2x dose, lights off, filters off, for 15 minutes, using a syringe to apply 5 ml per patch. SGD 22-28 per 250 ml bottle at local aquatic stores. Stabilise CO2 to a strong lime-green drop checker all day, improve flow to remove dead spots, and stock Siamese algae eaters (Crossocheilus oblongus) which are the few fish that actually consume BBA once it has been weakened by Excel.

Fix Flow and Dead Spots First

Red algae and cyanobacteria colonise spots where water barely moves — behind rocks, under driftwood overhangs, in deep substrate pockets. Rearrange hardscape if a chronic dead zone exists, add a Chihiros Magnetic Flow Maker or a second circulation pump at SGD 45-85 on Shopee, and aim for 5-10x tank turnover through the display. Vacuum substrate thoroughly during the next three weekly water changes to export the organic reservoir feeding the bloom.

Dial In Nutrients After Clearing

Post-treatment, lock in 10-20 ppm nitrate, 1-2 ppm phosphate and 10-20 ppm potassium using Seachem, APT or dry salts from Iwarna. Dose lean for low-tech tanks, EI-style for high-light CO2 tanks. SG tap water contains almost no N or P, so plants must be fed externally or they concede ground to algae and cyanobacteria the moment nutrient shortfalls appear. Maintain photoperiod at 7-8 hours during recovery, not the typical 10.

Watch for Recurrence in the First Month

Cyanobacteria leaves spores that can regrow within weeks if conditions regress. Test nitrate weekly and keep it above 5 ppm minimum, clean filters every two weeks for the first month, and do 30% water changes weekly. Any re-bloom in the same corner is a flow problem — fix the circulation, not the chemistry. Most Singapore hobbyists who relapse do so because they returned to a no-nitrate lean regime that created the vacuum in the first place.

Prevention for Tropical Tanks

Keeping cyanobacteria out long-term in Singapore’s warm water requires three constants: nitrate never at zero, circulation in every corner, and organic waste exported weekly via gravel vacuuming. Skim surface film with a Eheim or ISTA skimmer to prevent protein build-up that feeds it. Feed fish conservatively — one pinch twice daily for a 60 L community tank is plenty; leftover food rots fast at 29°C and seeds the next bloom.

Related Reading

• Algae in Fish Tank Complete Guide
• Black Algae Fish Tank Complete Guide
• How to Get Rid of Algae in Fish Tank Guide
• Brown Algae Fish Tank Complete Guide
• How to Control Algae in Fish Tank Guide