300+ fry are a lot of hungry mouths. I would set up an additional rot culture (back up is good) and harvest about 50% of each culture on successive days. i.e. harvest from culture 1 on Monday, culture 2 on Tuesday, and culture 3 on Wednesday. By Thursday culture 1 is nice and dense again, and so forth. A rotation like this with heavy harvesting keeps water in good shape via water changes and keeps the rots in a nice exponential reproductive state. In the lab, I take it a step further and maintain 4 seperate cultures, cleaning out the oldest culture vessel and thereby restarting one culture every week. Leaves me with excessive rot production, but if a culture fails it just doesn't matter 
