I suggested tail biting because he either has rot or has been biting. A lot of times they will bite when they are stressed (being in a small container, dirty water, aroudn other fish). I am mentioning this so you can keep an eye out for it, if, in fact, he is a biter. To me, it looks like bite marks.
I put aquarium salt in my fishes tanks after every water change. Just remember, always put back in what you take out, salt doesn't disintegrate so you don't want to hurt your fish with too much salt. I use API aquarium salt. If you're putting it into a tank with rocks, make sure to fully dissolve it before putting it in (the crystals can burn your fish). If you have a heater, higher temperatures like 80 or 82*F will help speed up the process.
I also hope he's not tail biting. The fish in my avatar is a HM and he LOVES biting off his gorgeous tail. I have no idea why he feels the need to do it, but otherwise he's very healthy. Most of the time long finned fish will bite their tails because when they turn around they can see it and it can annoy them. Especially if they're in those small cups or a small tank/bowl. I really wish pet stores would put bettas in their own small tanks.
well, it's better than fin rot and if he has been biting, he is now in a bigger and better home =[ sadly pet stores will never do that b/c it costs a lot of money to house each betta in their own tank...money rules over the quality care of the pets in MOST stores =[
Yeah, it's ridiculous. If they can't make a lot of profit on them, they don't care about them. You never see salt water or even fresh water fish in dirty water because they can make a lot of money on them.
Well all i can say is that AT LEAST the water at your petco looks clean. UGH drives me nutz when i see dirty cloudy water in all the cups.........i followed someone elses example and whenever i see one that is near death i take it up to the cash and leave it there.........GET THE HINT! UGH