Proper Nitrate Levels

Ammonia and nitrites should always be 0. Keeping nitrates at 0 would be good, but isn't realistic in all setups. A low level, less than 10 ppm, is the target of most marine setups--but unlike a planted FW tank, where nitrate is frequently supplemented to support plant growth, supplementing in a SW setup is unnecessary.

Nitrates don't harm fishes

According to my experience, nitrates even at 60 ppm do not harm fishes and some invertebrates like red banded shrimp, hermit crabs, and turbo grazers.
I had an 80 gallons aquearium with an Eheim 2229 ( wet/dry) and about 12 fishes along with the invertebrates that I just mentioned.

High nitrates are more of an indicator of total dissolved organic compounds and overall system health, such as PH issues. Some species are indeed less/more sensitive than others to DOC's and nutrient/metabolite buildup. For reef/invert systems, most target less than 5 ppm and they often keep sparse fish populations in those tanks to help limit nutrient input. For more robust fish systems nitrates are less of a concern; a 20 to 40 ppm reading is common and can usually be maintained/lowered with efficient skimming and scheduled water changes. While many fish are likely not harmed by moderate or even higher nitrate levels, IMO higher nitrates almost always go hand in hand with a low PH; and PH/alkaline reserve is a much more serious parameter that requires careful attention in all marine systems.