At least someone on Steemit has experience raising fry in a high density situation ;) Mine were also too young for a filter and I used to do daily 50% water changes between the fry and the parent tank. This is when you could get zeolite into that filter and using algae to clean the water of nitrates, etc comes into its own. The bonus is that the algae can be fed to the fry. Or tadpoles in your case