Try Mollys, the can live in almost any type of water even salt water, so ph levals won't be that big of a problem. As long as it doesn't change on them. They are easy to care for, they can live in a tank with just air bubbles if thats what you decide to do. Just no lid on the tank.

One inch a gal. is good but don't judge that inch on the size you get them at allow for their growth. If you want more fish get some bubble/balloon mollys, they don't get that big so you can have more for your space. They are really cute with their bubble bodies.

I have a 2 gal desktop tank in my kitchen with mollys in it, with just a air stone and I had them for little over 2 years now, so I know they do fine.

As for Bettas they will do fine too, Bettas labyrinth allow the anabantoids to breathe oxygen to a certain degree, making the chances of them suffocating in oxygen poor water slim. Meaning that they can live without a filter or even air bubbles if thats what you decide to do. That is of course why you see so many Bettas in just bowls and sold in small cups (peronaly I hate that) at the pet store. You know when they are shipped to pet stores some have been in the same small bag for up to a week.

But if you do go with a betta and a filter in the tank don't get a power filter for them do under gravel. I added a link if you are interested in them so you can see whats the best way to go.
bettas