What does Costia (Ichthyobodo) (Ichthyobodo necator) look like?

Excess slime coat (blue-grey film). Scratching/flashing. Rapid breathing.

How is Costia (Ichthyobodo) (Ichthyobodo necator) treated?

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.

FRESHWATER disease

Costia (Ichthyobodo) (Ichthyobodo necator)

Reviewed by the Fast Aquatics husbandry team · Updated May 2026

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.

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.