i have had same problem at times, believe it or not, fish can get gas! the sollution is to go to pet store and purchase some frozen shrimp brine, they'll know what you meen, it's not the same as the shrimp we eat. your fish will love it and it will cure the problem. expect it to happen from time to time. note; your fish might get smart and start faking it just to get the shrimp.