Glad it seems to be better now. The more frequent the water changes then the better your Betta will be 8)
I have a 37G, a 10G and a 7.5G. I did have a Betta with other community fish in the 37G (til he died), I have 1 Betta in the 10G and 2 Bettas in the 7.5G. All 3 tanks have filters in and I`ve never had any problems at all.
If you choose to have a filter then the main thing to ensure is that it`s not `bigger` than the tank needs, if you do you may find the flow is too fierce for your Betta to be able to swim near or cope with it.
I have plants in front of the flow from the filters which interrupts the flow a bit, therefore making it easily manageable for the Bettas to go near.
Bettas are pretty clever and will swim around the parts of the tank they feel happy with and do tend to get used to flows from filters....if not then they just don`t swim through the flow.