Fritz Mardel Maracyn Review: Erythromycin for Cyano
Cyanobacteria is one of the most frustrating tank problems because it is not really algae — it is a photosynthetic bacterium that laughs at blackouts and runs rings around manual removal. This Fritz Mardel Maracyn review looks at the erythromycin-based classic that remains the most reliable chemical kill for stubborn blue-green slime in planted tanks. Drawing on experience at Gensou Aquascaping in 5 Everton Park, the notes below cover dosing in Singapore HDB aquariums, plant safety, filtration rescue and sensible alternatives.
Understanding the Active
Maracyn’s active ingredient is erythromycin, a macrolide antibiotic that inhibits protein synthesis in Gram-positive bacteria and — critically — cyanobacteria. Fritz took over the old Mardel Laboratories formulation and kept the 200 mg per sachet strength. The powder dissolves cleanly in Singapore PUB water and does not stain substrate or silicone the way older dye-based treatments do.
Why Erythromycin Crushes Cyanobacteria
Cyano cells share a prokaryotic ribosomal architecture with bacteria, so erythromycin disrupts them identically. Within 48 hours of dosing you will see the mat turn pale, lift off the substrate in sheets, and float into the filter. Our cyanobacteria fix guide pairs Maracyn with nutrient rebalancing so the mat does not return two weeks later, which is the real failure mode.
Dosing the Planted Tank
Standard dose is one sachet per 40 litres daily for five consecutive days, with no water change between doses. On day six run a 25 percent water change and reinstate carbon. In our soft PUB water, erythromycin is remarkably stable so the five-day course gives a complete kill. Dose at lights-off to minimise photolysis. If your tank has measurable flow dead zones, reduce the dose slightly and run a small powerhead to keep the drug circulating.
Plant and Shrimp Compatibility
Maracyn is one of the few aquarium antibiotics that is genuinely plant-safe at label dose. Stems, crypts, swords and carpets all tolerate the five-day course without visible impact. Shrimp are a different story — Neocaridina usually survive, but Caridina show stress and occasional deaths, so shrimp-dominant tanks should dose at half strength or move the cherries to a holding tub. Snails are unaffected.
Filtration Impact
Unlike kanamycin, erythromycin is relatively gentle on nitrifying bacteria at the Maracyn therapeutic dose. Expect a mild ammonia blip on day three or four — test daily, and be ready with water changes and Prime or equivalent to bind any spike. The cycle usually recovers within a week post-treatment. Remove activated carbon and Purigen before dosing or the drug disappears in six hours.
Gram-Positive Bacterial Infections
Beyond cyano, Maracyn targets Gram-positive pathogens — certain Streptococcus species, some fin rot cases, and mouth fungus (Flavobacterium columnare despite the misleading name). For open ulcers from Aeromonas or Pseudomonas, erythromycin alone is ineffective; you need a Gram-negative tool like Fritz Maracyn Two or KanaPlex. Our antibiotics complete guide walks through picking the right one.
Singapore Sourcing and Pricing
A 24-sachet Fritz Maracyn box runs $42 to $55 at Iwarna, C328 and Y618. Smaller 8-sachet packs at $18 are fine for a single cyano outbreak but the large box is far better value if you keep multiple tanks. Shopee listings from authorised Fritz resellers occasionally hit $38 with free shipping. Avoid loose repackaged sachets from third-party sellers — the powder degrades with humidity exposure and potency drops sharply after two years.
Treatment in the Display Versus Hospital Tank
Because erythromycin is plant-safe and mostly cycle-safe, most cyano cases are treated in-situ rather than moving fish out. The exceptions are shrimp-dominant tanks, rare plant collections where melt risk is unacceptable, or tanks with scaleless species like kuhli loaches that stress more easily. For fish-disease applications, use a bare hospital tub set up per our hospital tank beginners guide.
Preventing Cyano Relapse
Killing the mat with Maracyn is the easy part — keeping it gone is the real work. Cyano thrives on low nitrate, low flow and organic-rich substrate, which describes most under-maintained Singapore tanks. Increase flow, hit 10 to 20 ppm nitrate target, siphon detritus weekly, and reduce photoperiod to 7 hours for a fortnight after treatment. Without those fixes, Maracyn is a temporary reprieve.
Verdict
Fritz Maracyn is the default chemical weapon against cyanobacteria in Singapore planted tanks and a useful tool for Gram-positive bacterial infections. It is not a one-bottle cure-all, and pairing it with husbandry changes is non-negotiable if you want lasting results. Kept alongside your quarantine medication kit, a box will see you through years of tank runs with few regrets.
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emilynakatani
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