Question
My two neighbors and I have small koi ponds. About 3 weeks ago all of the fish in one of my neighbors pond (he has 2 next to each other)were fine at night and in the morning they were all dead.  Today, the other neighbor had the same thing happen to him...   I'm very worried about my pond since we have no idea what went terribly wrong. My koi are my babies and I couldn't think of something happening to them...  What could be killing all the fish so fast?  Their ponds are about 250 gallons....

Answer
I'm sorry about the problems.  Do their ponds share anything?  I mean, do they use the same source water, get the same runoff, or otherwise trade anything back and forth?  I wonder if someone could have sprayed pesticides nearby?  One of your other neighbors maybe?  What symptoms did the dead koi have?  Did they test the water for oxygen, pH, ammonia, nitrite, and/or toxins like pesticides?  I suggest testing your water (both tap and in the pond).  Has it been really hot there?  Small ponds with large fish on really hot days will probably be low in oxygen so fish can suffocate.

Here's a list that I made entitled, "Why did my fish die?" that may give you some things to check into.

http://www.fishpondinfo.com/health4.htm#why

I hope your fish are ok!

Robyn