Something isn't adding up here (and it's not as simple as a test kit gone bad).
If you are doing daily 50% pwc with 40ppm water, but your tank stays at 180ppm, that means that at a minimum your tank is so stocked (or something died) that you are producing enough nitrates DAILY to equate to 140ppm. That's a fricken boat load of ammonia going in the take to keep nitrates that high.
This sounds a lot like what happens with those that do a fishless cycle and keep ammonia dosing high as the nitrite=>nitrate bacteria take hold. Basically, all the high dosing ammonia soon becomes nitrate, and because they are doing a fishless cycle without pwc, that they measure nitrates at 180ppm and do a 50% water change and still have 180ppm reading.
It seems this type of situation occurs because liquid test kits don't seem to do so hot at measuring high levels. So I'm thinking that your 180ppm reading is in actuality something like 360ppm or even 720ppm. Therefore even a 50% pwc leaves levels above 180ppm.
Try doing this. We know the test kit isn't totally faulty because it does measure the tap at 40ppm. So if you do enough PWCs, you should eventually get your nitrate reading at a level between 180ppm and 40ppm.
Stop feeding and do hourly PWC until you get a reading below 180ppm (hopefully very close to 40ppm actually). Then again, without feeding, wait over night and read nitrates again. See if they have gone up (which means something is decaying in the tank) or if they stay the same or lower (which should be the case when you've quit feeding for 48 hours, and/or you have a planted tank).
Once you've got a stable nitrate reading around 40ppm, resume feeding and see how fast the levels begin to increase.