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In a newly cycled tank there should only be trace nitrAtes, probably none. As the tank matures you could be looking at peaking at 20, which would be within OK parameters, i.e. tolerable for the fish, but any higher could be hazardeous.
Optimal levels of nitrate are around 10 ppm. That being said, somewhat higher levels are not dangerous if the fish in question are tropical. 40 ppm is tolerable; fish can acclimate themselves to higher levels. Of course, when you add a new fish that is not acclimated to those levels, you're asking for trouble. Also, high nitrates are usually a sign of an ill kept tank, and that IS a problem. Can cause major algae outbreaks too.
That is a good article and good advice from everyone.
It seems that my nitrate levels are a little high relative to the target. Would an appropriate short term remedy to do several water changes (say 25% 2 or 3 times in a week) until the nitrate level goes back down? Longer term solutions would seem to be to add some live plants and make sure not to overfeed/clean up any extra food real well.
I did water changes of about 10-20% on my overstocked 25g 2-3 times a week regularly. Nitrates stayed between 20-40 ppm and the fish did fine. We'll see how it does now that they moved into a 55g *grin*
You need to measure the amount of nitrate in your tapwater, and the amount that the nitrate level rises in your tank each week or month, and it will calculate how many water changes, and of what size, you need to make to get the water as low in pollution as you can.