You can figure about 10 lbs/gal (imperial gals) so 30 gal = 300 lbs plus tank & decorations, 55 gal = 550 lbs plus tak & decorations. The more of your stand touching the floor the more the weight is distributed, so if your stand is on 4 legs be sure to put something larger under them to help distribute the weight ( a block of hardwood or steel say 6" x 6" x 1/4 "). I'll try to find a previous posting here that was talking about this same topic and add a link