This is what i did... It uses a matten filter with an embedded water pump to provide Bio filtration. The water pump is so much quieter than the more common jet lifter. Plus it heats the tank to 80 just by running.
Water flow is right for Betta, a koralia nano is too much in my experience.
In a 20g the big block of form should be hideable. For a while I used it as the middle divider for a dual-betta 10g...
10g Planted Dual-Betta Tank w/Matten Filter https://www.aquaticplantcentral.com/...d.php?t=129826