The size of the grow out tank really depends on how many youngsters you get. I have ended up with over 100
at one time and that was just way too many for a small tank. Also the bigger grow out tank that you can put them in will allow them to grow bigger quicker, enabling you to start selling off the bigger ones making more room for the smaller ones. And believe me if they have the room (without competition) they REALLY rocket along. Any really young fry (about guppy fry size) are very tempting for guppies (and probably anything else besides BN's and Cory's), they will also go for the eggs even with the male guarding them. The best hatching and raising sucess I had was when I removed the eggs from the breeding tank and had them in their own mesh net cage (The guppies where giving the poor male a really tough time) still suspended in one of my tanks. The Babies stayed in the net until they lost their yellow and turned darker, then I let them out with the Cory's and because of all the room, weeds and hidy holes in this 4ft tank they never looked back. The hardest part was finding enough people to take the bubs once they hit the 4-5cm size. But as I was able to reduce the numbers the remaining youngsters grew quicker. Good luck with the breeding.