I wrote this for another website today, I thought I'd share it here as well.


Causes of bloating in angelfish (in order of treatability):

Severe constipation - Epsom Salt dosed at 1tsp/gal (30 minute bath), or 1 tsp/5 -10 gal (1 week extended bath) no food.

Hex/Spironucleus - Metronidazole doped flakes (and if you find it: Di-Metronidazole which is soluble in water), Parasite Guard (Tetra). Either one's efficacy is increased by raising the temperature to about 86F. But since there's a possibility of bacterial infection as well, I don't recommend touching the temperature, in fact lower it to 75F - 78F.

Nematodic infestation in its gut (capillarids are the most common type) - Levamisole (directly dosed into water), Fenbendazole (doped into flakes), Flubendazole (emulsify via a small quantity of acetone then dose directly into tank), Praziquantel (doped into flakes) or Parasite Guard (Tetra).

Systemic Bacterial infections causing Edema - oxytetracycline (doped into flakes AND dosed directly into tank) or kanamycin , (doped into flakes AND dosed directly into tank)

Egg-boundedness (in goldfish, you can massage the eggs out, you're out of luck in Angelfish..

Benign/malignant gut mass - euthanize then buy yourself a new one. (incidentally, I have many angels)

You can use "PAM" to bind any of the above drugs that will only work if fed with doped flakes. If your fish has stopped eating, then use ones that can be used in an extended bath. The symptoms of acute constipation occurs in a short time span and swells larger compared to those I've seen that was attributable to a pathologic condition.
If it's a systemic bacterial issue, there will be accompanying symptoms such as cloudy eyes, exopthalmia, severe mouth/tail/fin rot and/or red-streaking on fins and body to fin base perimeter.