Breeder tanks are best to be left bare bottom. That way you won't suck up fry that are hiding in the gravel. Also, you can see exactly what is in the tank. Instead of using a gravel vac, you just use a narrow pipe on a hose. You then syphon the debris to a waste bucket.
Some parenting fish such as convicts will chew up the food that you add and then spit it out so that the fry can easily eat it. It will end up as a frothy mess that looks like poo. You will have to resist the urge to keep the tanks spotless until the fry grow.