Is it true for larvae tanks as well?
And how can it be ammonia poisoning if there is no detectable
ammonia but deaths still occurs, at elevated nitrite levels?

i *think* (we all know i dont do chemistry) the real phrase should be nitrite is not that
toxic, implying a comparison between ammonia/nitrite yes if you have high nitrite levels
it will harm your larva, and yes you can get high nitrite levels without having ammonia.
what levels equal high depend on the fish, clowns can do ok in 1ppm ammonia or nitrite
though you shouldnt sustain either. several others wont do well at 1ppm of either.

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Richard Reynolds