And I don't know what it is or how to help them.
It started earlier this year, we lost several fish, one at a time, to something that basically made them rot. By the time they died the poor things were in a terrible state. We assumed a kind of fungus and treated the pond water accordingly, but it didn't seem to help. Over the summer, we removed the final two surviving fish, emptied all the water out, cleaned the entire pond, refilled it and then put them back. We then waited to see if any more fish died. They didn't, so we got a few new ones at the start of September and hoped for the best. Except the smaller of the two original surviving fish then died of the same thing and now one of the new fish has a spot on its side that looks like the start of the rotting illness.
Has anyone experienced this? Does anyone know what we should do to treat the pond? We have some pond plants and when we took them out to clean the pond in the summer I noticed a large number of leeches. I googled it at the time and couldn't find anything to suggest that pond leeches will harm fish, but could they? DH thinks our one surviving fish, who is really big, is maybe biting the smaller ones and those bites are then festering and rotting, but we have no evidence for this, other than that fish has survived while all the others from our original batch died.