My pond fish have this rotting disease

[ladybirdsanchez]

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.

You need to ask advice from a specialist. Is there an aquatic/pond store anywhere near you? I'd take a water sample with you and go and ask their advice. They might be able to test the water and see what's going on. Take photos of the fish with issues so they can assess it and hopefully they will be able to diagnose the problem.

For a pond fish repeatedly developing what sounds like ulcers or tissue breakdown, I’d be cautious about assuming it’s just fungus. Several bacterial infections, parasites, and secondary infections can cause lesions that look like the fish is “rotting,” and cleaning the pond alone wouldn’t necessarily remove the underlying cause.
I’d separate the affected fish if possible and check the water parameters carefully — especially ammonia, nitrite, pH and oxygen. Poor water quality can weaken fish and make relatively small wounds turn into serious ulcers.
The leeches are worth investigating too, particularly if there were lots of them. They can attach to fish and create small wounds, which could potentially become infected, although I wouldn’t assume they’re the sole cause without seeing the fish.
If you still have the surviving fish with the new spot, a clear photo of the lesion would be useful for comparison. Ideally, I’d also get advice from an aquatic vet or experienced fish-health specialist before adding more treatments, because repeatedly treating the pond for “fungus” can miss the actual problem.
I’d also avoid adding any more fish until you know what’s causing the lesions.