FRESHWATER disease

Skin + gill flukes (Dactylogyrus, Gyrodactylus) (Dactylogyrus + Gyrodactylus)

Reviewed by the Fast Aquatics husbandry team · Updated May 2026

Quick referenceSkin + gill flukes (Dactylogyrus + Gyrodactylus) are monogenean parasites that attach to gills + skin. Cause flashing + breathing distress. Treat with praziquantel (PraziPro).

Symptoms

Scratching/flashing
Rapid breathing
Excess slime coat
Cloudy eye(s)
Refusing food
Color fade
Pale or discolored gills

Cause

Monogenean parasites attach to gills (Dactylogyrus, egg-laying) or skin (Gyrodactylus, livebearing). Spread via fish-to-fish contact or contaminated water. Common entry: pet-store stock.

Treatment options

Praziquantel (PraziPro). Gold-standard treatment. 1 tsp per 20 gal. Single dose; repeat in 7-14 days for Dactylogyrus eggs. Reef-safe (won't harm corals).
Formalin bath. Backup. 45-min bath at 1 tsp per gallon. Use for fish that don't respond to prazi.
Salt bath. 5-10 min in 3% salt solution kills flukes on skin. Limited effect on gill flukes.

Always treat in a separate quarantine or hospital tank. Most medications are toxic to coral, invertebrates, and live rock biology. Consult an aquatic veterinarian for valuable fish.

Prevention

Quarantine new fish 30 days + dose prazi prophylactically. Avoid mixing tanks with shared nets.

Fatality + outcome

Low to moderate. Heavy infestations kill via gill damage + secondary bacterial infection. Light infestations are tolerable but stressful.

Full disease database · Symptom matcher · Q&A library · Glossary · QT timeline calculator

Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.