My opinion a large sponge filter.
I have not personally raised or attempted to raise betta but I have listened to presentations on the subject from local club members and seen their set ups.
Because of the hormones fry release into the water you need to do multiple water changes to achieve maximum growth. Feeding four times a day and feeding live foods will not help if you don’t do water changes 2 or 3 times per week.
At least if you have more than a handful of fry that is the case. If you have great success Justdes you most likely will need to cull down the numbers to a more manageable number?
I suggest nobody can raise 500 fry? . Not if you want to give them the best chance of growing strong.
Generally with betta fry a sponge filter is added at some point. Some breeders add it before the parents even breed, others wait a few days after the spawn is free swimming, some wait a week or two, some wait until they're much older.
Personally, I am about to spawn a pair and I have a very well established sponge filter in there. This one is adjustable so right now I have it set so the output is above the water and its just barely trickling into the tank. As the fry get older I'll increase the power a bit. Its also like that because I have the water really low.