Fish Die Within Hours After Turtle Removal: Residual Toxicity or Disease?

The Puzzle

Turtle removed, but new fish spiral and die in hours. Water tests "fine" (but which parameters?). The pleco survives—suggesting a toxin affecting only certain species, or a pathogen the pleco resists.

Investigation Steps

1. Test ammonia, nitrite, pH, KH, GH, chlorine, copper—not just "fine."
2. Run activated carbon in the filter for 1 week.
3. Do daily 20% water changes with dechlorinator.
4. Check for pesticide/herbicide runoff from neighbors.
5. Consider a full pond cleanout if all else fails.

Pleco Clue

Plecos tolerate lower oxygen and some organics better than goldfish/koi. If oxygen is marginal, they'll last longer.