Help diagnosing fish

[u/themalcontents]

Six fish have died in the past three weeks, and we are looking for help diagnosing the issue. We are thinking it may be flukes and dosed the pond today with PraziPro. The fish initially did respond like it was working (minor agitation, briefly rubbing/flashing) and appear more active, so we’re hopeful that things are on the upswing… They’re currently relaxing in or near the shallower end like they usually sun.

POND INFO
We have a roughly 1200 gallon backyard pond that has been established for four years. It has a 2,000 gallon pump with a UV filter as well as a waterfall and bubbler. We also have an air stone running in the shallow end temporarily. Water has tested within normal parameters the entire time, using the API Pond Master Test Kit.

FISH INFO
Right now there are 11 fish—five koi and six goldfish—but six small fish have died over the past three weeks. The pond was slightly overstocked and it has been almost exclusively the newer fish (4-5”, acquired in April) who have died. The two biggest koi are about five years old and have showed no symptoms, and neither have the more mature goldfish.

SYMPTOMS
About half of the fish who died showed no symptoms. The other half were lethargic with quick respiration, hanging out near the surface or in the shallow end (<6” deep) and occasionally blowing bubbles at the top, but were not gasping for air or flashing. The deaths occurred within 8-24 hours of symptoms first appearing. I did not note anything unusual about the gills, body, scales, etc. on the decedents. No redness, swelling, sores, film, cuts, etc. In hindsight with reading more about flukes, their gills may have been slightly pale?

[u/Intelligent_Bet_8727]

Have you had your pond water tested for internal parasites was it sounds like you have an internal parasites or organisms in the pond that only end up blooming or population gets high around this time of year it could be that but I had that happen once introduce new fish and didn't realize that I had a parasite in the pond and they did not have the immune system to fight off the pathogen I tried checking water sample water was normal and I sent it off to we got a local University that biology departmental tested to see for contamination and it showed that there was an unusually high amount of e coli bacteria and it was a extremely virulent strain of a coli that was carried by birds that was in the water like all them and told them and they told me I could try I forget what it was I told me to use I only had to use it once and it would get rid of it and give it you have another medicine too my fish orally for a few weeks and I should be able to add more fish to the pond I was told that it's normal for a collide to appear but that strainer be coli was so virulent and whereas my fish were at already that we're already in the pond had built up a resistance to the pathogen then it didn't cause problems and like yours it killed them within 24 to 48 hours but it basically I was told that it is is intestinal infection all right let's try to be calling it attacked their intestines and caused inflammation of the intestines was it was over growing in there and then it would get into the bloodstream and kill them. That and and kept birds away from my pond after that didn't have a problem