At some point the fish will outgrow the 10g and you will be having to do daily water changes.. breeding tank/fry tank a 10g is great - I have a few set up as well using 10gs, but I also have 30 gallons for growing out, as bigger space = longer to build up deadly conditions, more room = less aggression, bigger tank = faster growth, etc etc. At least from my research and teachings from many breeders with decades of experience. You can use 10 gallons, but it is wise to have multiple of them and spread out the fry once they start to get too big for one 10g, and you would need to up the water changes to daily if using 10gs as well - hormones and dead food build up a lot quicker in the smaller tanks.
As I said, imo (in my opinion) it is wise to keep the fry/juveniles down to a minimum when using smaller tanks... To grow fry healthy and to a good size the less you have in the tank, the healthier/better they will be.. why people use larger tanks to grow them out so they can have more babies than just a few.
Think of it this way - how many female bettas, or for the matter fish in general, do you recommend people to keep in a 10g? 5? 8? How are you going to stuff 20, 30 fish into that same space, and still have them healthy, growing appropriately, and not killing one another due to feeling "trapped" (which does happen with bettas)? If you wouldn't keep more than say 10 fish in a 10g tank in any other circumstance, why do it to the ones that actually need the space to grow, etc?
As mentioned in your thread about using a 10g -
Originally Posted by indjo
It's ok to use several smaller tanks (multiple 10gs) as long as you keep up with the water changes - smaller= less fry and more frequent WC.
Will need more than 1 if she wishes to keep more than a handful of babies and raise them..