Since I live in Germany, I don´t have access to most tubs, breeders in the US use. The Rack holds 12 breeder tubs (55cm x 40cm) and several hatchling and semiadult tubs (20cm x 30cm or 25cm x 35cm). 8 or 6 per level. The hatchling and semiadult tubs do have a lid. I use a heat cable for heating, since no flex wat heating tapes are available here. I hope you guys understand what I´m writing - it´s been a long time since I´ve learned English at school