I did some very quick
research because I am unfamiliar with undergravel and found some information that are pretty negative towards this kind of filtration
There is minimal mechanical and chemical filtration from undergravel filtration which might be the cause the water to be cloudy. I believe a decent mechanical and chemical filtration is needed to make your water clear at first since any waste released in the water would take longer to break down due to the small amount of bacteria. Website where I got this info is below
The Undergravel Filter Controversy
I also found out that using carbon with undergravel filtration is bad because the amount of carbon in the cartridges are so small, it only lasts a few days. Also, having carbon on top of the inlets of the undergravel tray would block the flow of water, thus limiting filtration. Website about this is below:
HowStuffWorks "Aquarium Filter Designs: Undergravel Filters"
That was just information I got from the two first websites I got when I googled undergravel filtration so it might be bias