your knife looks stressed to me... they only breathe that hard if they're stressed...
if it was because of low oxygen all your fish except your ck and poly would be breathing hard because they can both breathe atmospheric air.
how big is ur ck?
I'm moving my 12 inch into a 150 gallon indoor pond this wkend... he'lll be the only fish in there..
if yours isnt too big i would consider putting him in there with mine if you'd like to ship...
I can't really take him if hes over 1.5 ft though, the pond is only about 4-5 ft by 4-5 ft and 2 ft deep
pm me if youre interested. I can trade u two 3 inch baby bichirs if you want them
or some cichlids