How large is large?
What size pond, dimensions, gallons?
Filtration, water movement?
How long is this going on?
Any other fish/koi in the pond?
Underwater lights?
As David mentioned, do a check on your parameters.....
---
Problem: Second oldest Koi, a male, was resting unless fed for 3-4 days when posted. Koi appears to be back to normal behavior i.e. swimming around regularly.
---
Dimension: an oval, ~22' left to right, ~12' front to back, and ~4 to 4.5' deep
---
Filtration: surface skimmer box to a Dolphin Ampmaster pump to a ~75 sq ft planted gravel bog
---
Water movement: another surface skimmer box feeds another Dolphin pump to a hose dumping water back into the pond surface. Normally feeds a waterfall, but there's a leak, so a hose by-passes the waterfall.
---
Stock: ~50-75 goldfish, 7 koi from 9-16", and one ~4" black koi; 6-7 lily pots, and assorted pond plants (Parrot's feather, hornwort, duckweed)
---
Underwater lights: 4 lights hooked up into GFCI, 2 lights not working, other two under too much plant life to be noticed
---
Parameters: never checked. Pond is 19.5 years old. When water clarity consistently drops to less than ~2' or when we can, we vacuum the detritus form the bottom.
Yes, I'm overdue to do pond maintenance - plan to do that in a few days. Biggest issue is herons/crane, so a net was recently installed.