FWIW, the nitrite is not toxic in marine systems and generally is not worth measuring except for curiosity or to ensure a nitrate measurement is not beign interfered with by nitrite.
Nitrite and the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com
Summary
As far as has been studied, nitrite is not as toxic to marine fish as it is to freshwater fish. This difference is striking, with some of the most sensitive freshwater species being more than a thousand times more sensitive than typical marine species. Does this apply to all fish that might be kept in reef aquaria? I have no way of knowing. I have seen nitrite toxicity test results on only one marine species that we normally maintain in reef aquaria (Figure 3). Nevertheless, there is no reason to think them more or less susceptible.
The same can be said of nitrite toxicity to marine invertebrates; few studies have been done. Those that have been tested suggest that lethal doses of nitrite are fairly high (i.e., tens to thousands of ppm nitrite). Biochemical and developmental effects have been observed at lower doses, however, and there is no reason to suspect that the most sensitive invertebrates have actually been tested.
Aquarists obviously can decide for themselves, based on the data provided above, what level of nitrite to allow in a reef aquarium, or alternatively, at what level to take some action to reduce it (e.g., a water change). Fortunately, nitrite levels in reef aquaria are almost always far lower than would cause concern from a toxicity perspective. While a new aquarium's nitrite levels might reach up toward 10 ppm or more, such an aquarium should not yet contain sensitive organisms. Once the nitrite is below 1 ppm, there is little reason to be concerned.
From my perspective, nitrite and the nitrogen cycle may be interesting phenomena to monitor during a reef aquarium's initial set up, but nitrite does not warrant routine testing. I haven't tested any aquaria for nitrite in years, despite setting up several reef aquaria in that time. I simply do not think that nitrite is worth the time and expense of testing, unlike ammonia, phosphate, alkalinity, pH, calcium, etc. It is my opinion that many things other than nitrite concentration are more worthy of reef aquarists' attention.
