 Fish gulping and blowing bubbles at surface
I have several Koi in my pond and several goldfish. The Koi are doing well but one of the goldfish is gulping and blowing bubbles at surface. Its belly also looks swollen. I have read numerous articles and am still unsure if its swim bladder or low oxygen (surely this would affect all the fish). I have just thrown in some cooked shelled peas but they sink to bottom and it is impossible to see if the affected fish is eating them. I have tested water quality and readings are good. Any suggestions plz? | 
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