It is most likely an internal problem.. There's not a lot on slowed breathing, but the main cause seems to be a parasite, ammonia/nitrate, or simply temperature shock. Are you sure it's not shock? There's nothing to do about shock but wait. Some fish can be a lot more sensitive than others.
Pick up some tetra parasite guard. Wal mart should have it..
If you have a fish store you might be able to find Metro+ or Seachem's metronidazole... Tetra parasite guard has the ingredient too.. If you can't find any of those, settle for Epsom salts... There are other medicines with the metronidazole, but they contain ingredients that can be toxic to betta, better not to try...
Edit: don't bother with aquarium salt, it could put uneeded strain on gills. Like I said, Epsom. Wal mart should have pure Epsom, if not any drug store will...
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Last edited by Olympia; 06-01-2012 at 10:19 PM.