Methemoglobinemia, more commonly called brown blood disease (which we have discussed in detail in other posts), can happen to most living organisms if nitrate gets in the blood stream. The question remains at what concentration in water does nitrate pose a threat to fish, specifically the ones we're interested in culturing. The study lists some species at different stages of development.

Nitrate toxicity to freshwater and marine fishes increases
with increasing nitrate concentrations and exposure
times (Trama, 1954; Westin, 1974; Colt and
Tchobanoglous, 1976; Rubin and Elmaraghy, 1977;
Kincheloe et al., 1979; Brownell, 1980; Tomasso and
Carmichael, 1986; Pierce et al., 1993; Scott and Crunkilton,
2000). Furthermore, nitrate toxicity can depend
greatly upon the cationic composition of the solution
(Dowden and Bennett, 1965). As in the case of aquatic
invertebrates, freshwater fishes appear to be more sensitive
to nitrate toxicity than marine fishes.
Well this is getting complicated. And Tilapia are capable of both freshwater and marine environments! What is also interesting is this study would lead you to believe that all fish have low tolerances for nitrate the younger they are up to being an egg; however, Tilapia eggs are impervious to nitrate and even ammonia meaning they have no toxicity at that stage of development either.

Also, it has been my observation the feeder goldfish are raised in such poor conditions that the ones that survive are quite adapted to high nitrate, nitrite, and ammonia levels. They have been Darwinized to have good genetics for this kind of thing.

Anyway, this is pretty amazing data and there's quite a bit of it. Thank you for sharing. It's going straight into my archives!