I don't test for it and I don't recommend that anyone else do.
I discuss nitrite here:
Nitrite and the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com
from it:
How is Nitrite Toxic?
Nitrite can be toxic in a number of ways. Freshwater fish rapidly take up nitrite through their gills, leading to high levels in their bodies. In freshwater fish, nitrite taken up through the gills can compete with chloride for the same uptake proteins, so in some cases of elevated nitrite the fish can suffer from chloride depletion. It has been observed that some freshwater fish (e.g., bluegill; Centrarchidae: Lepomis macrochirus) do not take up chloride via their gills, and these species are notably resistant to nitrite toxicity.
The internalized nitrite then causes a number of internal disturbances, including loss of potassium from certain tissues (such as skeletal muscle) and the oxidation of hemoglobin into methemoglobin, which reduces the blood's oxygen carrying capacity. This can cause reduced tissue oxygenation, hyperventilation and heart rate increases. Many other biochemical pathways become altered as well, including steroid synthesis, vasodilation (blood vessel enlargement) and changes in internal levels of ammonia and urea. Nitrite detoxification in freshwater fish is accomplished by direct nitrite excretion and by internal conversion of nitrite into nitrate.
Marine species are less susceptible to nitrite toxicity because chloride (at 19,350 ppm in seawater) outcompetes nitrite for the same uptake mechanisms. Nevertheless, it is possible for some marine fish to take up nitrite via both their gills and their intestines after swallowing seawater. For example, when exposed to 46 ppm nitrite in seawater, the European flounder (Platichthys flesus) takes up 66% of its nitrite via intestinal routes.24 Further, its internal nitrite concentration was found to remain below the ambient nitrite level in the water. At these concentrations, there was some alteration of internal biochemical parameters (such as an increase in methemoglobin levels from 4% in nonexposed fish to 18% of hemoglobin in exposed fish). Nevertheless, there were no mortalities under these conditions, and the difference between this result and what is often observed in freshwater fish at similar nitrite concentrations is attributed to differences in their internal nitrite concentrations.
How Toxic is Nitrite to Fish?
For the reason described above, nitrite is considerably more toxic to many freshwater fish (Table 1) than it is to most marine species (Table 2). The data in these tables are primarily the LC50, which is the concentration at which 50% of the test organisms die (24-h LC50 is the concentration that kills half of the tested organisms within 24 hours). As Table 1 shows, some freshwater fish can die at nitrite levels below 1 ppm. This toxicity is the reason many aquarists worry about nitrite in aquaria. It can be a significant problem in freshwater aquaria. Tests in marine species, however, showed the toxicity to be much lower. None of the thirteen marine fish species for which I could find nitrite toxicity data had LC50 values below 100 ppm, and half had LC50 values of 1,000 - 3,000 ppm or more.
Death is, of course, a very crude indicator of toxicity. An aquarium's nitrite level should not come anywhere close to the LC50 value, because less severe toxicity can occur even at levels below that. In the previous section, I showed data on one marine species in which biochemical effects could be detected at levels well below concentrations that caused death. We saw, for example, a rise in methemoglobin at values as low as 46 ppm nitrite. However, the point remains valid that marine species are orders of magnitude less susceptible to the effects of nitrite than are many freshwater species. The marine aquaculture industry often uses a rough guideline that the safe rearing level of many compounds is a factor of 10 or less than their LC50.
In examining ammonia, nitrite and nitrate toxicity in marine species, one might think to look at the effects on larval fish to see if they are more sensitive. In examining the incidence of the larvae's first feeding after hatching, and the 24-h LC50, it was found that for seven different marine species, only ammonia was found to be toxic at concentrations that might possibly be encountered in aquaculture facilities.25
Table 3 brings out the distinction between freshwater and seawater organisms most clearly. In these tests, two fish and one shrimp species that are able to live in both freshwater (or brackish water) and seawater were tested for toxicity at different salinities. At least for these three species, it is clearly shown that nitrite is much more toxic in freshwater (or at lower salinity) than in seawater, even to the same species.
In the only published article that I could find showing toxicity tests to typical reef aquarium fish, Tom Frakes and Bob Studt exposed tank-raised clownfish (Amphiprion ocellaris; Figure 2) to nitrite concentrations ranging from 0 to 330 ppm in artificial seawater. Two of five fish died after a few days at 330 ppm, giving an LC50 not appreciably different from the other species listed in Table 1. At 33 ppm (the next dose down from 330 ppm), the fish were lethargic and breathing with difficulty, but otherwise experienced no lasting problems. At 3.3 ppm nitrite no effects were observed.
One of the difficulties with interpreting toxicity issues, as related by hobbyists who claim to have seen nitrite toxicity in marine fish, is the possible presence of ammonia. In any aquarium with elevated nitrite, the ammonia level also may be elevated. Since ammonia is known to be very toxic to marine fish (LC50 value below 1 ppm), on the aquarist must ensure that the observations are not flawed by such contaminants. In all of the toxicity tests described above, nitrite is added directly to the seawater, and ammonia would not be expected to be present at significant concentrations, whereas in aquariums the levels of the two materials are not independent of one another.
