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 LC50.30
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
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.
Figure 3. Like most reef aquarium fish, the susceptibility to nitrite toxicity of this longhorn cowfish (Lactoria cornuta) from my aquarium has not been established.
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.
