I always suggest 20 or below will be OK. Just keep 0 as the goal and you`ll be OK. Here is an excerpt from a link on nitrates.
"Nitrate is an ion that has long dogged aquarists. The nitrogen that forms it comes in with foods, and can, in many aquaria, raise nitrate enough to make it difficult to maintain natural levels. A decade or two ago, many aquarists performed water changes with nitrate reduction as one of their primary goals. Fortunately, we now have a large array of ways to keep nitrate in check, and modern aquaria suffer far less from elevated nitrate than did those in the past.
Nitrate is often associated with algae, and indeed the
growth of algae is often spurred by excess nutrients, including nitrate. Other potential aquarium pests, such as dinoflagellates, are also spurred by excess nitrate and other nutrients. Nitrate itself is not particularly toxic at the levels usually found in aquaria, at least as is so far known in the scientific literature. Nevertheless, elevated nitrate levels can excessively spur the
growth of zooxanthellae, which in turn can actually decrease the growth rate of their host coral.
For these reasons, most reef aquarists strive to keep nitrate levels down.
A good target is less than 0.2 ppm nitrate. Reef aquaria can function acceptably at much higher nitrate levels (say, 20 ppm), but run greater risks of the problems described above.
There are many ways to reduce nitrate, including reducing the aquarium's nitrogen inputs, increasing nitrogen export by skimming, increasing nitrogen export by growing and harvesting macroalgae or turf algae (or any other organism of your choice), using a
deep sand bed, removing existing filters designed to facilitate the nitrogen cycle, using a carbon
denitrator, using a
sulfur denitrator, using AZ-NO[SIZE=-1]3[/SIZE], using nitrate absorbing solids, and using polymers and carbon that bind organics. "
