Yes I agree - but a 15/20 even 10 gallon tank can be much to big for fry which will get lost in all the water & be unable to get their food. To avoid this just drop your water level to a few inches and then raise it a few weeks after your fry are born.
Are you just asking to house your bettas though?
I do spawn in my grow-outs, using a wide surface area of water & good filtration before fry are born because the (PG Rated here) 'leftovers' of a spawn are actually toxic.
Please ask lots of questions if you are unsure of yourself before spawning your bettas & remember MALES & FEMALES DO NOT MAKE GOOD "TANK MATES"
Nes.