It seems awfully deep into his body for fin rot...I'm not saying it's not, but not as a first sign of trouble...Why do you think it couldn't be a heater burn? I googled & some pics look similar...I've never had that issue...But an antibiotic seems like the best course of action right now...Jenna, which antibiotic treatment are you using? I don't know which 1 or 1s I'd suggest...
Also Jenna, in your 25g, did he have hiding spots under wood or rocks? I "think" that's how heater burns happen, when fish try to hide as best they can even if they have to cram themselves wherever they can, like under or next to the heater. I agree with tl, he seems pretty big for a 25g...It's hard to judge size from pics...
My fingers are crossed for your pleco! Please update us tomorrow, we're worried about him!