like petluvr said. its part of the filtration system. both ends of the tank there is a square box with slits. as water is pumped into the tank it fills up the extra water drains into the over flow. the water goes down into a sump which then gets pumped back up to the tank.the biggest tank i myself would put a betta in would be a 55 gallon.and i may know w alot of stuff about fish but i truly dont know what an over flow is.could you tell me?
IMO, it depends on what you want for decor, filtration, heat, etc. If you're fine with a pinch of substrate and a tiny plant (fake or live), they can live in a vase if you change the water daily. If you want some live plants, other decor, heat and filtration, I can't see going smaller than a standard 10g.im wondering the world has said many different things about betta size tank some say half a gallon some say 10 gallon.but what do you say?