This is an algea eater. His name is Catfish. Currently, he's been living in a 10 gallon tank with a few guppies. I kind of feel bad for him (her) because the 35 gallon tank I raised him in sprung a leak and he's had to adopt to his current cramped quarters.
He's a big sucker. He's quite friendly and doesn't spook easily unless he's being gawked at by an observer. But I can tap at the glass where he suctions his mouth and he will stay put. Even while I siphon the water for filtration with a tubular contraption that works much like a vacuum cleaner -- and nobody likes the way vacuum cleaners stir everything up -- Catfish will still remain still as I vacuum underneath him. That's bravery.