Uv will have no effect on nitrate as far as I know.
Do you gravel vac when you remove water?
The reason I ask,
When I had two plecostomus 46&36 cms. in an 860 litre tank, I had to do two changes per week, 200 & 100 litre. Most of that water was siphoned off of the gravel with a gravel vac. That's less than 50% no problems. Sometimes I needed to do more!
There were other fish in tank 11 clown loach and always 20+ Cory.
The filters were 2 FX5 fluvals which I did one every other week, so two cleans per month per filter, 4 cleans total. Complete strip down new wool etc.
What do you call a water change, some people consider just taking water out of the top to be sufficient.
This was copied from another post I made;-
Water change should be re-titled syphon-filter or gravel-vac to remove water and detritus from your system. Remove a clean bucket of water from the tank and use this to clean filter media, sponges etc. at this time check impeller and clean if necessary, replace filter wool with new clean material and replenish tank with new correctly acclimated water with the correct parameters.
So I suggest for one plecostomus,
Keep on vacuuming gravel to get most of the waste out, 125g with a plecostomus probably will need 3-5 buckets per clean, removed by vac. Twice per week, dividing week up. You may need two buckets of clean tank water per filter.
All the muck in filter and on tank bottom is causing the nitrates, my tank didn't have enough plants to remove all the nitrates, plecs eat plants. They are heavy waste producers.
How many of which type filter is fitted?