Koi with Large Body Holes: Disease, Toxins, or Cement Pond Issues?

New pond owners: two koi died with large purplish holes; one survivor has similar sore. Pond is cement, 3.5' deep, with waterfall, skimmer, two aerators. Algae killer used repeatedly.

Possible Causes

• Bacterial infection (Aeromonas/Pseudomonas): Treat with KoiZyme, MelaFix, 0.05% salt, injected antibiotics (Baytril).
• Heavy metal toxicity: Test for copper, iron. Dechlorinator binds metals.
• Algae killer toxins: If contains copper/herbicides, can burn fish. Do water change + activated carbon.
• Cement pond pH: Uncured cement leaches lime → high pH (>8.5) burns fish. Acid wash or line pond.

Action Plan

1. Test water: pH, hardness, ammonia, nitrite, heavy metals.
2. Partial water change with dechlorinator + salt.
3. Add fresh activated carbon.
4. Stop algae killer; use barley straw, plants, bacteria.

See Koivet.com for koi health details.