Eventually the spawning aquarium is going to be too small for the fry. If there are hundreds of fry, this can be as soon as 10 days to 2 weeks. On average, I move most fry around 4 weeks of age. Because I use small spawning aquariums, my preferred method is to remove the heater and float or set the spawning aquarium into the larger grow out aquarium and leave it there for an hour. Sometimes I will take a small clear cup and catch a few fry, write down the number and place the fry in the grow out aquarium. I do this over and over until the spawning aquarium is empty of fry. Fry do die occasionally so counting them at this age, does not mean there will be that number when all is said and done. Still, I prefer to get an idea on how many fry are in a particular spawn. If you do not care, you can also float/set the aquarium the spawning aquarium and then pour the entire contents into the aquarium. Grow out aquariums can be a 10 gallon or larger. I like 20 gallons since I can still manipulate them but if the spawn is very large, you may need to split up the fry again as they age.
Important: Make sure the water in the “grow out” aquarium is the same composition, dechlorinated and heated to the same temperature. Shocking the fry at this point can cause an outbreak of velvet or ich…if they don’t just die on you. Set up the grow out aquarium at least a couple of days before you plan to move the fry so that you have time to test the heater. They can malfunction and cook the fry or not keep them warm enough. It is also a good idea to have live plants in any newly set up aquarium as it helps to keep ammonia levels down. Remove any uneaten food and any dead babies to keep ammonia levels down.
Snails: Snails are great at eating left over food, dead babies and algae. Ramshorn or mystery snails are wonderful additions to any grow out aquarium. Ramshorn snails do reproduce quickly so if you want non-reproducing, choose mystery snails. Mystery snails do lay eggs above the water line once in awhile bit it takes two to tango. Ramshorn snails do require two snails to reproduce but all can lay eggs so the reproduction rate is much higher/faster. If you ever get overrun with them, put a fish food bottom feeder pellet in a small glass bowl and come back the next morning. Most of the snails will be on the dish in the morning and you can remove as many of them as you like.
Baby bettas start to take air in from the surface somewhere between 10 days and 3 weeks. It is very important from 9 days on to keep scum off the surface. This can be done by using a small sponge filter. If you do not have one, setting a plain white paper towel on the surface of the water twice a day will keep most of the scum off and allow the babies to breathe properly. Personally, I only use sponge and mechanical filters in my grow out aquariums but not spawning aquariums.