My undergravel filter flow has slowed right down. It's a resun dm-800 120 litre with the UGF built in. The gravel is regularly cleaned and the filter plates at the bottom of the tank were taken out and cleaned not two months ago. It's barely even trickling when it should be a proper flow. It's not airstone powered the water is pulled up by a grey box that must contain motor and then sent through the filter box via a power head. The water then comes back down into the tank through the other side.

It may be clogged or something, you know, small gravel fragments may have filled the holes in the plate. Either way, just get it exchanged, throw it out and never think about using a UG filter again. They really aren't suitable for goldfish and won't do anything but collect more dirt in the gravel than necessary. They were pretty common at one point, but should really be avoided in my opinion