Hello and thank you for your inquiry.
Unfortunately there is no simple answer to this other than as low as possible. Different species will have different tolerances to different levels of certain chemicals/elements in the tank. While one species may do fine with nitrate levels up to 10ppm before it becomes harmful, others may have a tolerance up to 20ppm, 30ppm or even higher levels. For this reason it is best to try to keep your nitrate levels as low as possible, or at the very least under 5-10ppm.
There are many different products that can help reduce nitrate levels within the reef tank, but generally the best way to lower the levels (or keep them low if they are not high) is to keep up with normal water changes, don't overfed or overstock the aquarium and employee a good protein skimmer on the tank.
If there is anything else we can do for you please let us know.
Marine Depot Customer Service