I keep mine in at least 5.5g nowadays simply because I prefer to heat and cycle the tank. However I had great success in the pat with two 2.5 gallon tanks on my desk at work. I went in on weekends to clean their tanks or cleaned during slow times at work.
My house gets too cold in winter to keep unheated tanks as we don't run the heater so warm. I was tired of 100% water changes, and my bettas went into shock and died.
So now, I fishless cycle the tank, then heat it and change only about 20% of the water weekly or a little less often. I treat with water conditioners, age, and heat the replacement water to match the tank's.
I also like 5g because I can use a 25W heater with a thermostat instead of the little heaters that just raise the temperature a few degrees. In out cold house, that wouldn't work.
5g or more are just easier on me and my fish to handle because I cycle the tanks, don't have to remove the fish, clean out the tank, try to get the water back just 'so' and then return the betta to his tank.
Bottom line, I believe that at least 5g is best and healthiest, but if a keeper can keep them healthy in 2.5g (they have at least a little room to swim around in this size) that's fine with me. I did, so I know it can work even if I don't enjoy that method anymore.
Last edited by H20Gardner; 09-28-2011 at 02:24 PM.