I don't have a lot of practical fishkeeping experience, but I'm getting a 20 gallon long soon, so I've done some research for stocking a tank that size. Even if I hadn't, I'd say that's greatly overstocked!
Personally, I'd get rid of the pleco and the Siamese Algae Eater
for sure. I'd also choose between the male betta, 5x female bettas, or 2x Bolivian rams as the feature fish for the tank and return the others.
I know you say the male and female bettas get along fine now, but you're asking for trouble keeping them together in a tank that size. Males shouldn't be housed with females. Honestly, the reason the male isn't bothering them is probably 'cause he's stressed out by all the other fish in the tank. (Ultimatebettas.com).
Also, I'd probably return the female guppies. Having males and females together means babies. Plus, the tank would still be overstocked.
I'd also return the Neon Tetra
, since he wouldn't be comfortable without a school.
This the stocking I'm planning for my 20 gallon, just to give you an idea of how much to fit in there:
- 2x Kribensis
- 3x cherry barbs
- 4x Sterbai Corydoras
- 6x harlequin rasboras