@Dan: I hope the algae still avoid your tank on his new location.
@somewhatshocked: I understood the word slightly. But slightly or not, misleading still is misleading. It only tells that I´m not completely wrong.
If a substance is fed to animals doesn´t mean it can´t have some side effects if it is fed constantly. Ethoxyquin may be not a natural substance but still is fed and it has side effects..
The ADI for oxalates ascertained over time ranged from 0.3 to 3 mg/kg bw, but due to suspected different bioavailability in different species (so hardly transferable to human), and due to the fact that the daily intake is often higher than these ADI values they aren´t used. No reliable ADI exists, as far as I know.
The LD50 is 375mg/kg bw (ascertained with rats, source: Page 5, article 11
, unfortunetaly in German).
Supposing the weight of a fish species would be 3 grams, that would mean half of the population of this species would die by eating 1.125 mg ca oxalate (according to your link duckweed contains 2 - 4 % ca oxalate, let´s take the middle: 37,5 mg duckweed (with 3% ca oxalate) would be needed to achieve this value). Well no 3 grams fish would probaly eat that much duckweed all at once plus due to the minor solubility propably even more duckweed would be needed.
But I never talked about direct intoxication. I even noted duckweed should be pretty save if fed only from time to time. I only warned of constant, everyday consumption which may lead to some health effects.