I have 4 in a 60" long 100 gallon. One 8 1/2" male, and 6 1/2", 6", & 4" females. There is a 7" pleco, a 10 1/2" Royal CK, and a 12" CK in there too. I do two 25 percent water changes a week and have twin under gravel filters hooked up to a pair of Powerhead 350's, and run two Emperor 400's on it. I also have two 23" foam aerator strips and a 9watt Turbo Twist UV sterilizer. My sevs cruise around with their mouth closed. Maybe try airstones?
I have a 9" female gold sev in a 55 gallon with an 11" pleco and a 12" oscar. No breathing heavy there either. But, I have two 18" foam aerator strips in there also and one Emperor 400.