I have typically kept my bettas in 5 gallon tanks. Last year I used my 40 gallon breeder tank for a new betta and he lasted a few days before he died. The tank was set up correctly and it was 95% full of water. Is that too much? I would assume not..but that was my only experience putting a betta in a large tank. It has been a long time but I often wonder what happened. He appeared fine but maybe he wasn't.