What are your fish going to do with 2 more gallons anyway? You can already keep 2 Bettas very happily in the 3 gallon.. and the filter in it is better than the walmarts cheap one one so I bet the water quality would be better even in a smaller space.
Keeping an almond leaf per gallon should lower the PH to begin with.. and then add new leaves when you do water changes. You're the one that posted the aricle on Almond leaves