You might also want to post this in the reef forum!
Ammonia is toxic at pretty much any level, I can't speak for Nitrite but I can tell you that the amount of Nitrate's that are acceptable depend to a certain extent on livestock. I have never been able to get my nitrates below around 5-10ppm. Toxic levels I am told are above 100. However most corals do not tolerate high Nitrates, whereas fish can endure (If they have to) higher amounts.
HTH,
Rob
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