Q: Dear Faith, I have noticed lately that my blue male betta has only been using one of his pectoral fins, I wanted to know if this is a bad sign or if his "arm" is just tired? lol. Thanks!
A: Is it a bad sign? Maybe not, but let me put it to you this way: Mr. Betta is not keeping his little pec idle because he is tired. Although bettas on occasion will swim with the least amount of hard work (cause they are, like, real lazy and stuff ;) ), so sometimes will only tread water with one little fin and no more, this is rare and always very temporary (say a few minutes).
I assume your betta however has been on a one pec regimen for longer than just a few minutes. This indicates that he sustained some pec damage somehow. Either another fish picked at him, or he got stuck somewhere (behind a rock, in a plant, behind a corner filter), or he might have a slight bacterial infection.
As mamma Faith always says (geese, maybe I should stop calling myself Mamma Faith, I am aging myself). Hum. Let me rephrase. As beautiful young (very young) mermaid who swims effortlessly with bettas says ( ah... much better) better be safe than sorry. So in doubt, treat him for a little bacterial onset. I suppose a little Bettamax and a drop of vitamins could not hurt. No, no, no, you cannot put a cast on it LOL. Why? Cause bettas are slippery and the cast will slip right off. LOL. Also you have a male and males are notorious for playing tough and not wanting to wear a cast, or take care of their health for that matter. Cause they think they are, like, indestructible or something.
Ladies, do I hear an "amen"???
To conclude, I would make sure there is nothing in the tank/bowl that can hurt him, look for anything he could have gotten stuck in, behind or under. If you locate such an item, remove it. If it is another fish, remove it. If all looks fine, then not much to do other than a bit of bettamax and some vitamins. And let time do the rest. Your betta's life is not in danger and he will probably recover soon.