a single goldfish in the 20H wouldnt be very expensive really but goldfish can range from 15-$40 for really nice ones. plus you said you cant support the wight so i guess 20 gallon is out of the question for now.
if you find you water is very alkaline and hard, try out guppies, or endlers, or maybe a few planties. bettas can handle any water really.
if you find you water is very alkaline and hard, try out guppies, or endlers, or maybe a few planties. bettas can handle any water really.