if they were covered in slime they are old dead, so it happened a while ago. the
muck on the bottom has anaerobic fermentation going... and koi sit on the bottom,
even root on teh bottom and toxic gases are released from that muck.
Ponds really gotta be cleaned in fall. cant let em go until spring. it is a bitch
cause it is always sooooo cold. but with the netting and my veggie filter I dont
have muck cleanup.. didnt when I had a greenhouse over my big ponds. this spring I
expect to see a lot of dead fish in the big ponds, none in my new koi pond. sigh.
the other thing to consider is if the fish werent in good shape and/or had a load of
cooties that could have been the cause. Ingrid

(MC) wrote:

Two of my Koi died last weekend and floated to the top. I am in
Chicago and have kept the ice from freezing over with airstones and a
heat source. I am wondering if they just got sick (they were covered
in slime) or if something in the pond caused it. The water is quite
brown from all the fall leaves. I tried to keep out as many as I
could, but a large number (realtive to pond size) got in. Next fall I
will cover the pond with netting. Could this have affected the fish or
does the water just look ugly to us?