We have a dilemma in our water garden. Our goldfish are dying, one by one, every few days I find another floating, we've lost over 10 in the last few weeks. This is a well established pond (about 8 yrs old, for 6 years it was a koi pond but I found homes for them), 3500 gal, the water quality is good, no new fish have been introduced (earlier in the spring I did), I have looked over the fish that have died and they look fine, I can't see anything wrong with them, other than they are dead. We have frogs, about 5 that live in the pond and they are fine, the plants are healthy. I'm at a loss, does anyone have any ideas of what might be going on?