20 gal is only ok for keyhole cichlids and possibly rams but I wouldn't recommend any others. That size tank is fine for these mixed with a school of tetras and some clean up crew, either catfish or crustaceans. I recommend these fish and tend to steer clear of live bearers. I have kept them before and tend to find the breed like crazy and die like crazy. By the way I'd go for mechanical without fail. With that size tank you can have an HOB. With regard to substrate, if you do go for kuhli loaches like I said in my fist post and you get sand prepare never to see them again as mine seemed to tunnel in it all the time in my previous tank ( also a 20 gal)