This is one of the problems with buying SH from a LFS. The specimens they buy are generally touted as "tank-raised", but the tanks are generally open-system vats that are connected to the ocean water, so these SH come in with all the same pathogens as wildcaughts (WC). SH are very prone to getting bacterial infections (usually Vibriosis), so as Flower mentioned temperature is a HUGE contributing factor. At 75*F, the bacterial count in a closed system goes thru the roof, which is the reason behind the lower temps for SH.
The best thing you can do to prevent bacterial infections is to lower your temp a bit. 74*F should be the absolute max temp in a tropical SH setup, but they'll tolerate cooler temps, as low as 68*F for tropicals. Keep an eye on the SH, and if you see a lesion forming (check the tail as well), you'll definitely want to drop the temp, get it into a hosp tank and treat the SH. Furan 2 is the current antibiotic of choice.