If they have at least moderately hard water with a pH of 7.0+ then livebearers and some small loaches for the bottom would be good.
If the water is somewhat soft-moderate with a 6.0-7.0 pH, then most schooling fish such as barbs, tetras, cories would do well.
Most gouramis, bettas, and plecos do fine in either conditions; they adapt better than many other fish.
and good luck!
Current fish (includes all 3 running tanks): Polypterids, severums (of which I raised from eggs), gouramis, plecos (gold spot ones & a gurupa), loaches (zebras, yoyos, & a kubotai), macculocchi spotted silver dollars, an African butterfly fish, & Ctenopomas (a leopard & an ansorgii)