Jacky,
>Thus the question is:
>why do you have to still try to achieve such a low P (if you are
>going to recycle the water into the fish tank)

A: The key is that we are not recycling the water back to our fish, we
are discharging the water to the stream. Trout are cold water fish and
if you were to recycle it back to the trout, you would have to cool it.
The fish do not require such low P levels.

Paul R. Adler, Ph.D.
USDA-ARS
45 Wiltshire Road
Kearneysville, WV 25430 USA