Reptile & Amphibian Forums

Thanks in advance for anyone's help. I have a wonderfully healthy all-season 500 gallon goldfish and koi pond that has been healthy for 7 years. Today, I walked outside and found 1 dead goldfish and all the rest of the fish gasping for air at the surface of the water. By the time I was able to remove the dead fish, get some salt into the tank and start draining, 2 more had died. I replaced about 25% of the water in the pond and hit it with a heavy dose of chlorine and amonia remover. The remaining fish are now swimming again but still behaving eratically. They are very lethargic and appear to be "gasping" as they swim. Some are moving better than others.

This completely took me by surprise. We had a very heavy rain yesterday that could have washed some contaminant into the tank, but that is all I can come up with for what might have caused this. 2 weeks ago I had an exterminator out who sprayed the perimeter of my house... could this have gotten in there? I was always under the impression that if insect poison got into the pond, I'd pretty much have a pond full of dead fish within minutes. Any suggestions on where I can go from here to protect my remaining fish?

I had just noticed 2 new fish in the pond (lots of spawning going on) a few days ago. I'd hate to lose them.

Curiously, the small black "mosquite-eater" fish that are also in the pond (around 50 of them) are all acting perfectly normal.

What the heck is going on?

It does sound like you have had some of poison enter the pond but if that were the case you would have more of the little fish dying too i wouldve thought.
As this seems to have happened after a big downpour then the rain itself may have caused a ph crash, though of course it would take a lot of rain to do this, i have known it happen. check your ph and see if it has altered. In the meantime i would do the 25% water changes with dechlorinator for a few days and hopefully this will right the problem. If not, then you may need to look elsewhere.
hope that helps.