One foot pleco! Sum of a ...... I wouldn't doubt that either. I had a 2 foot eel in a 90 gallon and that still wasnt big enough. Although he had caves and and would dig around or go threw tunnels etc. I dont want to make you feel bad or anything but often times fish stores sell fish with wrong information. Some are very good, some well.... the responsible thing to do is to give him away even for free to someone who is able to house them. They are awesome. I used to have one too. But they grow and grow and grow. It's a learning experience but dont give up. My advice would be keep around 8 to 10 Cory's. Sex your convicts. Keep one male. If you have 2 or 3 females that's okay. They'll bully and chase but not seriously harm. Get ride of any other males. I've seen my convicts "fight" they puff up they're chest with my jack dempsey female, but no actual fighting they just go back and forth and then one leaves or they both run away lol. Cory's are smart enough to run away. In bigger numbers they should be okay. If your angelfish looks fine then welp leave him. If you had stocked your tank too much it is possible you could've had a infection hit. Hey, it happens. If they looked visibly sick or infected they may have been. Maybe consider a 75 in the future as convicts can also get up to 6 inches. In a 55 bow that's pretty big, plus all the other fish. More space is always better! All the best luck. I wonder how it's going?