I posted this under tank mates, but need some advice:

My little guy's dorsal fin is torn down the meat and he is having a hard time swimming. Looks like some bad damage to his tail. I think I might have to put him down. I have been watching pretty carefully, but i was gone today, and I didn't witness the incident. I have a spare 10 gallon tank i can quick set up as a hospital. I do have antibiotic for my Bettas on hand. Are they safe for Oscars? (Tetracycline and a Sulpha)

Don't put him down just yet, let's try and help. In a lot of cases, you are better off having an injury than a disease, at least you know what you are dealing with. Just to monitor the fish and medicate the tank with let's say melafix for the time being, this is a good medication for injuries. It is the fish eating okay? Is it breathing heavily at all? Anyway, get back to us with some answers. Sometimes fish can have quite serious injuries and still recover perfectly okay.

Just curious. Can a fish grow back any fins that have been torn & tattered. And, is treating my tank on a routine basis with melafix and pimafix an alright thing or a bad thing? Thanks!

Yes, they can grow back fins unless the roots have been destroyed completely. Well from my experience at least. If a damage went very deep the fin might grow split at times or won't look as beautiful as before, but I'd always try.
Like Penn said already, it's amazing from what they can recover at times.

I have never tried melafix or pimafix, but at least for melafix I seem to remember it's something with only natural ingredients rather than a real medicine? Would think it shouldn't hurt then probably. But would still not use it for prevention because I simply don't see it as necessary. As long as the water and fish are fine, I see no need or use.

I think it really does depend on how bad the damage is. Fins grow back very well indeed. However, very bad fin rot could cause irreversible damage. Please post some photographs so we can see. Make sure the water is in good condition, this always helps with healing