I used to use an internal corner box filter, (those things with the polyester wool in them), in my fry-raising tank. But, discovered that an undergravel filter is fine, providing the airflow up the up-lift tube isn't too fast. Guppies in particular like slow-moving water and don't like swimming, "up-stream", so to speak. I've never personally had any problems with any fry getting stuck in gravel.