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Polished water and green spot algae

While playing with PO4 and NO3 in the water column I came across something
interesting.  At the time I remember being surprised, but after-the-fact it
seems pretty obvious.  

At one point the water in my tank suddenly took on that crystal-clear,
polished look where you can see every O2 bubble and the outline of ever
scale on an SAE.  This occurred when I had reduced PO4 to 0.05ppm and NO3 to
less than 0.2ppm.  This combination appears to allow the tank to remain
phosphate-limited but apparently doesn't provide enough excess nutrients in
the water column to grow much water-borne algae.  During the time period
that I maintained these conditions I saw no visible algae, nor did I have
any green spot algae on the glass surfaces.  I've been able to reproduce the
effect in a tank with active plant growth.  The downside is that stem
plants, which depend on these nutrients in the water column, don't
appreciate being starved.

As soon as I upped the NO3 level to 1ppm the water clarity reduced slightly
and the green spot algae returned (like most of us I have to buff it off the
glass weekly).

michael rubin 
michael at rubinworld_com