How should i convince my dad to let me get a bigger fish tank?

i currently have a 15 gallon tank with 5 guppies, 1 neon and 1 glofish. i am hoping to get 4 more neons and 4 more glofish, and 1 of my guppies is pregnant. its her first time so i dont think that she will have a lot of babies but i know that even if she has only 5 then we will need at least a 20 gallon right, to keep her and her female babies and for the males and the neons and glofish right? i have the mom in a breeding tank right now, (one of those floating cages with the bottom for the fry in it) so how can i convince him that my tank is to small?