Koi Fish kill. But not all of them

Question
Hello, we have a fishpond with about 15 koi, bought all from baby 3in state, and now they are all about 14 in. give or take. Recently, one of them swelled up huge. Looked like it could have been egg impacted or something. WAYYYYY abnormally large belly, besides that, looked healthy, ate, and swam like normal. Today, we left in the morning to my grandmas, everything was fine, we got home at  nine-ish, and five were dead, including the swelled fish. The others are fine so far. But it is only been a matter of hours. The only different things I can think of is I fed them a  piece of bread last week cuz we were out of food. This was after the fish was already swelled. But one piece of bread divided between 15 fish of 13 in wouldnt kill em right? Could the swelled diseased one have died, and unleashed deadly gases?

Answer
I'm sorry about your koi.  The enlarged koi probably had dropsy or perhaps a problem with his/her kidneys.  That is usually not contagious.  It may be worsened by other problems such as poor water quality which could have impacted the other fish.  The bread should not have killed the fish.

See this section on my site entitled, "Why did my fish die?"
http://www.fishpondinfo.com/health4.htm#why
Has it been extra hot there?  If so, the oxygen could be low.  How big is your pond?  You have a lot of koi.
For a lot of fish to die fast, it's usually related to chlorine, ammonia, poisoning, etc.  I suggest testing the water for pH, hardness, ammonia, and nitrite to rule those out.  I wish I had time to suggest more things right now.

Good luck!

Robyn