Right now, I have 2 Black Tetras, one Silver Molly, and one Pleco inside of a 10 gallon tank. I want to add 5 Neon Tetras to have them swim in shoals. Would this cause a problem such as overcrowding? Also, whenever I leave the room where my fish are and come back and turn the lights on, the Pleco continues sucking on the side of the aquarium then stops. Next, the Pleco zooms to my aquarium heater and sucks on it, but doesn't move around. Is this normal?