Great, thanks to know, Aus! I thought it seemed highly unlikely that a fish could have asthma...but I have treated him for everything else, and I couldn't figure out what else it might be...so thought I'd consult. The water chemistry is good, tested it and all parameters are normal. I've had him not quite a year, and he was an adult when I purchased him, so I know he's on the old side for a betta, but this came on very suddenly, one day he was fine, the next day he was lying on the bottom of his tank gasping. At first I thought it was a heater leak or something of that nature, but nothing, and I've since used the heater in another tank with no problems.
So...I treated him with salt, then tried Jungle Fungus Cure, then I tried anti-parasitics that were rec'd by a good, high quality LFS, then one more time with the Fungus Cure (it's a nitrofurazone based antibiotic with an antiparasitic and PPM as well, so it usually hits just about everything) before I finally gave up and decided that if whatever he had didn't kill him, the medications would. It's been over a month now...so whatever it is isn't acutely lethal...and apparently it's nearly impossible to kill it off. Water changes have no effect, nor does changing out his tank, all his plants/decorations, or his heater.
As for long-term ammonia problems, I wondered about that as well. When I purchased him he looked large and healthy, and other than a bout with Ich (which I treated with coppertox...ugh...if only I knew then what I know now!) and a very minor case of fin rot, he's been happy and healthy until this recent episode. But, just because he looked healthy when I bought him, doesn't mean that he wasn't unhealthy at some period before. Either way, he's in a quarantine tank, and I put a few crumbles of fungus cure in there with each water change because he seems to gasp less with it...Not sure what else to do, I know he shouldn't be living on the meds, but it's either let him suffocate, or have a shortened lifespan by constant low-level meds.
Any suggestions? I'm up for anything! :) Thanks for your help!