I'd take him/her back and get something else that would function better in that size of tank. I have ten of them in a 29 gallon. When I only had six in the tank I was having aggression issues and the fish were very jumpy. I added four more and they are doing ten times better than before.
10 gallon: Red male betta & mystery snail
15 gallon: Blue male betta & two mystery snails
29 gallon: Ten swordtails