Quick referenceCostia (Ichthyobodo necator) is a microscopic flagellate that causes a blue-grey slime coat in freshwater fish. Highly contagious; treat with formalin or salt + heat.
Symptoms
Excess slime coat (blue-grey film)
Scratching/flashing
Rapid breathing
Color fade
Refusing food
Lethargy
Cause
Ichthyobodo necator flagellate. Outbreaks triggered by: cold water (under 70°F), poor water quality, recent shipping. Healthy fish in warm clean water rarely develop costia.
Treatment options
Heat to 86°F. Costia cannot survive above 86°F. Raise temperature 1-2°F per hour. Hold for 14 days. Add airstone for oxygen at high temp.
Aquarium salt. 1 teaspoon per gallon for freshwater (avoid scaleless fish). Maintain 7-10 days then water-change down.
Formalin / Quick Cure. API Quick Cure (formalin + malachite green) at standard dose. Effective and fast-acting.
Combination protocol. Heat + salt is the safest combo. Heat alone for shrimp/scaleless tanks.
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
Maintain temperature 76-82°F. Quarantine new fish. Don't overstock. Provide stable water parameters.
Fatality + outcome
High in stressed fish - costia breeds rapidly and damages gills. Untreated outbreak kills 50-80% of stock in 2-3 weeks.