You need to test the water and see what your parameters are. Without more info everyone would be guessing wildly.
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Approximately 83 10-20 inch koi. No new fish, same stock for 6 years, imported from Japan. Filtration is on a 3600 gallon pump bio-pellet filter (I know I need more filtration and flow). I have an 1800 gph pump on a sanyo waterfall filter but it's leaking somewhere and is currently not running. The bio-pellet filter is fed through a bottom-drain. Never did water changes except topping off the pond every 3 or so weeks due to evaporation. Still need to get a test...
so no one thinks this could be khv ?
Clean it weekly, full of bio-pellets. Has a leaf catcher prior to entering the bio-pellet chamber if that matters. I don't exactly remember the size of the chamber but I'm 5'7" and could probably fit in there.Well with that stocking rate and the lack of filtration I would bet it is a water quality issue, if that pump is pumping 3600gph (and most pumps do not pump what they are rated at after figuring head loss from pipes and fittings) you are looking at about a 4 hour turn over rate. Most koi keepers shoot for a one to two hour turn over rate at the least. How large is your filter(how much and what kind of media) and do you clean it regularly? Get that test kit because if the koi that died looked ok one day and dead the next you could be looking at a mass die off if this is not corrected soon. I would be looking for high Ammonia or Nitrites first and then making sure your KH is high enough to keep your PH stable and support what filtration you do have so you don't have a PH crash. You are either going to have to get rid of some of the fish load or inceace your turn over and filtration or you will just continue to have issues.