Hi there,
Undergravel filters need just that, gravel. They don't really work with sand as its so fine. I remember older aquariums ~(before powered filters exploded onto the market) with 2/3rds UG filter with gravel, and then a divider with the remaining 3rd containing sand (that had to be hoovered).
UG systems are basic, and if well maintained, can be quite effective, but external/internal filters, or even sponge filters, are a better bet in my opinion.