Reverse flow under gravel filters are nice in a planted tank, but it's not very useful in an uplanted tank.
Normal flow undergravel filters will trap the junk in the gravel, waiting for you to siphon it back out. If you're blowing the junk through the gravel and back out again, then you're using too much power. You can either use less powerful powerheads/air pumps or cap off one tube and only use one of them.
If this is a new UGF
, it will take time for the bacteria to develop in the gravel, you'll need to be patient and not overstock the tank before it's ready to handle a full load.