If the substrate is any bigger than sand sized, its a detritus trap. It's not what its made of that causes the problem, its the big gaps that uneaten food/fish waste can fall into and not be filtered out of. With a water change 4 days ago and a skimmer running full time, you really shouldn't have nitrates that high unless you have been seriously overfeeding. As a short term, vacuum your CC
real good and do another water change, and slow down on your feeding. Speaking of, how much, how often, and what are you feeding?
Long term, (again) dump the UG, change your substrate, and ditch the canister filter unless you are going to be real commited to cleaning it real often. I know it sucks to quit using equip. you paid for, but lots of us have been there. I started with a W/D and CC
and had nitrate problems I couldn't solve (thanks LFS
!). With DSB
my 'trates stay below 20. Wish I had started with it, would have saved a lot of money and effort.