1-2 degrees per hour should suffice, though I typically try and change 1-2 degrees in no less than 4 hours (but that's because I deal with cold water fish).
You may have internal parasites. Normally, parasites are noted by white, stringy poop. HOWEVER, I have also seen this bubble-type poop in a fish of mine that just got over a nasty parasite. It seemed that it was in the later stages of the disease (he's fine now), but don't quote me directly on it as it could be a first sign.
What other fish do you have in your tank? You could go buy some Jungle anti-parasitic medicated food and give it a shot. It's one that can be used as "prevenative medicine", if you're not too sure.
75 Gallon High-Tech Planted Community
3.5 WPG using T8's
Fluorite w/sand mix
Rena Filstar XP2
Pressurized CO2 w/SMS122
29 Gallon Goldfish
4" Red/White Ryunkin
4" Orange Fantail