Koi Lethargic in 300-Gallon Pond: Overcrowding or Illness?

A koi lying listlessly on a ledge in a 300-gallon pond with four koi is a red flag. The pond is severely undersized for koi, and overcrowding is likely the root cause—or a major contributor.

The Space Problem

Koi experts recommend:

• 1,000 gallons for the first koi
• +100–200 gallons per additional koi

Four koi need 1,300–1,600+ gallons. Your 300-gallon pond provides ~20% of minimum volume.

Consequences of Overcrowding

• Chronic high ammonia/nitrite (even with filtration)
• Low dissolved oxygen, especially at night
• Stunted growth and deformities
• Suppressed immune systems → disease outbreaks
• Aggression and stress behaviors

Immediate Diagnostic Steps

1. Test water NOW: pH, ammonia, nitrite, nitrate, KH, GH, dissolved oxygen, temperature.
2. Check for other symptoms: Clamped fins, flashing, red streaks, white spots, fungus, swollen eyes.
3. Measure fish: If any koi >12 inches, the pond is critically overstocked.

Short-Term Triage

• Large partial water change (30–50%) with dechlorinator and temperature match.
• Add aeration: air pump + air stone(s) immediately.
• Pond salt 0.1% (if no plants/snails) or 0.05% (with plants) to reduce stress.
• Stop feeding until ammonia/nitrite are zero.

Long-Term Solutions

1. Rehome koi: Find a larger pond (1,000+ gal) for at least 2–3 of them.
2. Expand pond: If space allows, enlarge to 1,500+ gallons.
3. Convert to goldfish pond: Goldfish tolerate higher densities better than koi.

Contact a Koi Club

Local koi clubs often offer free pond assessments, water testing, and microscope scrapings. They're your best resource for hands-on help.

Your koi's lethargy is almost certainly environmental. Fix the space/water quality, and recovery is likely—if caught in time.