Question
I live on Kauai in Hawaii.  I have a 3,000 gallon, 2-foot deep liner pond which has been established for 4 months.  Water quality is perfect -- Nitrates and nitrites are virtually non-existent, ph perfect, hardness and buffering perfect, no ammonia.  I use a Laguna Max-Flo 2900 pump with a fountain which provides lots of aeration, and a Laguna Pressure-Flo 3200 filter with built in UV sterilizer.  This basically means the entire pond is circulated through the filter every hour.  I have water lilies, calla lilies, elephant ears, papyrus and thalias in the pond as well as lots of underwater anacharis and lots of green algae on the bottom and sides.  I started out with 10 koi and 6 orandas.  I'm now down to 8 and 4.  Problem -- once every few weeks I find the koi and goldfish lethargic and gasping at the surface in the morning, when they were fine and healthy the evening before.  One or two fish die each time this happens. I do a 10% water change every week and a 30% water change once each month.  I do a 40% water change when the fish start gasping and they usually recover within a day -- but not this time.  They're still distressed and suffering.  Any suggestions?

Answer
If they seem to be gasping, there is either an actual shortage of oxygen or there is something inhibiting their gills from processing oxygen.  I'd say it's safe to assume there is no shortage because of your filtration system.  I wouldn't rule out some sort of toxin or poison (such as fertilizer) just because they stopped ailing after the water change.  They could just be more resilient or not have made contact with a concentrated amount.

It could be parasites that attach themselves in the fish's gills, such as flukes.  Is every single fish gasping?

If every one is gasping, look for something in the water.
If only some fish are gasping look more closely for parasites.

If another fish dies and it doesn't gross you out too much, take a screwdriver or knife and pry the gill cover back.

If the gill is a consistent brown color, it is something chemical, like ammonia burn or fertilizer runoff.
If the gill is still pink but has inflamed spots, it is flukes.