Thanks everyone. I didn't mean to start a debate, lol. I have experience in freshwater so it's not like I'm going into this completely clueless (at least I hope I'm not). Mine were going since the summer time and had to take them all down recently because we moved. My boyfriend and I have been talking about a saltwater tank for a while but because I'll be the one maintaining it. I wanted to see what I was getting myself into. I've always known bigger was better but just wanted to make sure 20 long wasn't pushing it to be too small. I've heard of people having 3 gallon -5 gallon salt tanks but I don't feel that adventurous, lol.
10 gallon: Red male betta & mystery snail
15 gallon: Blue male betta & two mystery snails
29 gallon: Ten swordtails