Inverts are much fussier than fish so people aim for zero and generally get jumpy at anything above 20.
Denitrafication goes on dep inside your live rock, inside a dsb if you have one, or in a denitrator. Generally the aerobic areas of a tank outvolume the anaerobic areas so there is more area/oppotunity to produce nitrates than to reduce them. This is wet drys and biowheels for example are termed nitrate traps. Nitrate is the end product , as it is in live rock, however these things produce really quickly in amounts no denitrator can hope to deal with (except a whacking great, hard to run sulphur bead device or similar). Also as they're entirely aerobic, and nitrate CANNOT be reduced in an oxic enviroment (bacteria are present but the energy physics of the chemistry won't work) they typically build up nitrates quick