take about 50 or so plastic containers (unused slurpee cups and the like work fine) and a sharpened crochet needle (or a fine-tip awl if you have one). use the needle to punch ten or twenty holes in the sides of the cups, and float them in a half-filled 20 gallon tank, heated and filtered, and put a betta in each. this will allow them to be separated and grow individually, and the punch-holes will allow in-and-out flow of clean, warm water to each cup. be careful not to let the cups fill too high (leave about 3 inches of air space at the top) to prevent them from jumping from one cup to the next and spreading unholy carnage.
EDIT 1 - this only needs to be done as the babies get to be a month or two old, when they start to show their aggression.
EDIT 2 - keep up with daily or 48 hour water changes, depending on the number of fry to reduce the presence of stunting hormones.