Hi all,
I must admit, before today, I had no idea that urea had such a low toxicity to fish. If I'd known more about animal physiology it would have helped, but the penny should still have dropped a lot earlier.
I've been regarding urea and ammonia (NH3) as toxic at much the same concentrations, which looking back on it that didn't really make any sense.
I'll see if I can find a salmonid urea reference. If I can, it is more likely to give us more idea of the actual level at which urea (or more correctly the ammonia from its catalysis by urease) becomes an issue.
We would still have the uncertainty surrounding ammonia uptake by plants, the amount of oxygen in the system, the amount of ammonia oxidising microbes, pH etc.
cheers Darrel