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phosphate limited/rebuilding substrate?


    OK, time for my 3rd post of the day.  I'll be moving my tank next week,
and I have a few questions as I'm going to have an opportunity to change
some things around.  My tank specs: 100 gallon, wet/dry w/25 gallon sump,
350 gph flow, pressurized CO2 injection on Sandpoint controller, 280w of
VHO, substrate heating cables, 1.5-3" of fine gravel with a little bit of
Flourite mixed in, Dupla laterite in bottom 1/3rd of gravel.  Water is very
soft out of the tap, and is raised to 5 degrees KH and 2-3 degrees GH with
sodium bicarbonate and calcium hydroxide.  First off, I'm suspicious that my
tank may be phosphate limited.  I'm consistently getting results of 0 with a
Seachem test kit with good reagents, but am having a moderately serious
algae problem.  I was seeing bad hair algae, but am starting to see what may
be cyanobacteria (green slimy stuff on the edges of my plants and attached
to the hair algae that breaks off very easily and floats around the aquarium
until it sticks to something else).  My nitrates are consistently low or
zero, but my ammonia tends to be elevated.  All of these are done with
Seachem test kits, so I'm not too sure about the ammonia reading (I find the
kit very difficult to read, as the little disk is rarely a consistent
color).  If this is the case, what can I do to add more phosphate to my tank
to reduce my nitrogen levels?  Simply feed more?  If I'm wrong, what is
going on.  I'd really love to get rid of the algae.

    Second, I'm thinking of making the substrate a little richer, as I'd
like to be able to reduce the amount of available nutrients in the water
column (for the algae reasons mentioned above).  The gravel I have now has
about a year's worth of aging, so plenty of fish poop (no kitty castings
:-), and will probably be left alone if I don't rebuild it.  What will give
me more nutrients in the substrate without seriously raising the risk of
nutrients leaching?

As always, thanks in advance for any help.
Justin Collins, who is enjoying pancakes in sunny Bellingham, WA