I've been wanting to upgrade my betta's tank. It's currenly a standard 2.5 gallon with a sponge filter and mini heater. It's ok, but it feels so tiny to me, it's hard to make it look nice. Today mom and I went to Michaels and they had 12" diameter "bubble" vases for $20. It's a sphere that calculates out to about 3.5 gallons. The other option is that I could get glass cut and build a custom cube aquarium for him. I'd probably make it a 10" cube, which works out to about 4 gallons.