I have the PETCO 2 gallon hex that comes with the power filter 5. All told it is a nice little tank and a good betta home. I was not completely satisfied with the filter cartridge for the power filter 5. I did not want the carbon in it and I did not like the fact you had to replace the entire thing at once. Any bio filtering bacteria that grew on the filter would get tossed out.
My solution was to ditch their filter cartridge and to use a pair of Fluval filter sponges back-to-back in its place. I cut it extra large and crammed it in so water would not bypass the sponge. It is inexpensive and should last a long time. It has no carbon and it keeps the water crystal clear. When it does come time to replace, I can swap 1 of the sponges and leave the other so bacteria can restablish. The sponges have a lot of surface area to help bacteria growth.
I have been running it for a week or so and so-far-so-good, we'll see how it does long term. I am optimistic. So far, I have noticed a decrease is brown diatoms, which is good.