Bettas prefer soft water, and I don't know if you're familiar with them, but I have found that glass catfish might prove to be a suitable tankmate. They prefer softer water and are a shoaling fish, given their name, glass cats are literally see-through, I have found that my males don't usually bother them because they are rather "drab" I'm assuming due to the lask of colors. Glass cats are on the moderate scale however, acclimating them might prove difficult, but due to their peaceful temperment, they have always been the best tankmates for my bettas. In your case however, I dont think your tank is large enough though. as i said, they shoal. if i were u, i would invest in a larger tank.... stray away from guppies, as previously said, they can be the best or worst tankmate, it depends on your bettas temperment. and mollies. they prove rather bossy and will harass the slow-moving betta. if u wanna stay on the safe side, i would go with platies. they do however, prefer harder water. be sure to pm me for questions
Hmm, well I have a lot of fish...