Is the Koi flashing (scratching itself against the pond walls or rocks) or sitting at the bottom? Is it floating with its tail up? Is it floating, gasping for air by the waterfall? Is it only one Koi or a group of Koi? If the Koi were dead already, how was it swimming?
Koi’s behavior can tell us many things. If they are flashing or sitting at the bottom, Good chances are they got parasites. If they are gasping and floating, they may have gill damage caused by parasites or bacteria. Or it can be simply a lack of dissolved oxygen.
Read more in our koi health diagnosis guide.