I keep some "Orange Saddle" Cory hybrids and I just feed more sinking food and feed more often. Any aquarium fish will be healthier in pure water conditions. So, if you aren't removing and replacing most of the tank water weekly, you really should gradually work up to that point. Just keep the water clear of dissolved pollutants from the fish waste and feed a little more and the fry will grow quickly and be healthy.
The best thing you could do for them would be to keep the water parameters stable and pristine. You could do small frequent water changes, that way the water temp doesn't go way off and the water quality remains pristine. Fry tend to be more delicate than their adult counterparts which is why water quality is key.
Provide cover for the fry to make them feel secure.
If the water quality is pristine, they are feed a high quality diet, and feel secure and NOT stressed, they will grow quicker.