IMO, anything bigger then a 2.5 gal should have a filter and be cycled first. That's a lot of water to have to change out once a week. That's just my opinion. If you don't have a heater think about getting one as well as a thermometer so you can make sure the temp is staying steady. Your betta would definatly appreciate and benifit from a steady temp. Other then that everybody else said the rest of what I was going to. Good luck with your new boy/girl and welcome to the wonderfully addictive world of bettas!