Hi everyone,
I have a roughly 1700 gallon, two tiered pond with a waterfall and aerator that I treated for string algae late yesterday afternoon. This morning my husband found one of our 6" comets dead. Yesterday afternoon at feeding time, she was happy, active and feeding, and when I did an autopsy this morning, there were no signs of injury or illness at all. Gills still reddish, no fungus or skin wounds/infections, scales intact, no parasites, lots of food in her gut, swim bladder inflated, no telltale streaking of a systemic infection, etc. All of our fish get annual external parasite checks and we have never had an issue with flukes or other nasties. It's like she just died with no identifiable cause.

We had a total of 3 comets plus 8-9 of their babies from this spring (all under 2"), 8 koi, and various frogs and snails in the pond. It's heavily planted with anacharis, and water hyacinth, and water parameters are perfect. pH 7.6, Ammonia 0, Nitrite 0, Nitrate 0-5. Water is clear and plants are growing very well. Water is well aerated, and no other fish are showing any stress or sickness.

However, we did find one very odd thing in the bottom of the upper pond yesterday, where our DOA lived. We found a mostly picked clean squirrel skull. There were no other bones or evidence that a whole squirrel fell in, and the water is shallow enough that, (a) a squirrel could have gotten out on its own, or (b) we would have seen something before now, or (c) rotting squirrel would have produced an ammonia spike during the weekly chemistry tests. So how the heck did a squirrel head end up in my pond and could this have contributed to comet's demise?

I am thinking it may have been O2 depletion in the upper pond overnight from the algae treatment since it is quite shallow. Does anyone have any other thoughts on what could have caused her to die so suddenly?