i've had an altum angel 5" tall, 3-4" long in a 20gal with various mixes of tankmates (usually loaches, tetras, mollies) for almost 6 months and while many of his tankmates found their final moments on earth there he has survived and matured. i tossed him in a 55 for a little bit where he loved the swim room but as long as he has bubblestones to play in he stays pretty distracted