I have a relatively new 55 setup (about six months old) and it's only partially planted, with just a few small and medium sized plants (so far). However, one of the filters on it is from a very "seasoned" aquarium, so I have never had a problem with starting the nitrogen cycle going. However, while my my ammonia levels are always at <0.25ppm and nitrites are always at 0.0ppm, I can only keep my nitrates below 40ppm with weekly 50% water changes. I recently bought a bunch of fish at Petco's 50% off sale a few weeks ago, and although I moved two large plecos from that tank to a different tank (actually lowering the bio-load of the tank despite the many added smaller fish), the same readings continued, and I lost some black neon tetras and a rainbow. I suspected it may be that they were not accustomed to the higher levels of nitrate, which weakened their immune system and they succumbed to an Ich infestation of their gills. (I say the gills because, while there were very few spots on the black neons first--mostly on their fins--and then on other fish, I don't think what was visible was enough to cause mortality.) Of course, I did a heat & salt treatment on the whole tank and got rid of it (my own fault for not quarantining new fish, right?), but I suspect it was the higher nitrate level that was the underlying cause for the initial stress on the new fish. From now on I will be checking my nitrates and doing a significant water change before adding new fish (AFTER quarantining them, of course).
Although I have been out of the hobby for a long time, I consider myself a relatively experienced aquarist, but I don't recall ever having this much trouble with nitrates before. Maybe I've just forgotten about it, but I'm hoping that as I add more plants and they begin growing well, they will take care of the nitrate "problem".
The bottom line is that without other stress factors being apparent, I can only suspect the higher nitrate level as being the cause that stressed the black neons (who weren't used to it) to the point that they succumbed to the ich (which most likely came in with them or the other new fish in the first place). Had they been accustomed to the higher nitrate levels to begin with, I don't think there would have been a problem, since I carefully and slowly drip-acclimated all the new fish. Also, since the majority of the other species didn't suffer such a high mortality rate (just the one rainbow), I suspect that either black neons are more sensitive to higher nitrate levels or they are more vulnerable to ich.
That's just my recent experience regarding nitrate levels, so take it for what it's worth. (2 cents?)
"May the Fish be with you."