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Plenum tank consumes KNO3 (Or - Can't keep it up!)

Hi All,

Just passing on an observation, not making any recommendations here.  My 60
gallon planted has one 175W MH light and pressurized CO2 with a reef-style
plenum under laterite/clay enhanced gravel.  20% water changes every 1-2 weeks
and enough CO2 to keep the pH below 7.  KH is about 4, hardness is about 120
ppm.  I am using the PMDD regime but have to dose the KNO3 separately because
the tank demands so much more of this component to keep any measurable nitrate
in solution.  I mix 1 tablespoon of KNO3 per 500 ml of water.  I've been
adding 12 ml per day for the past two weeks and when I measured it today -
zero, zip, nada.  I added 40 ml to get the level up to about 8 ppm.  I'll bump
the daily dose to 20 ml.  Meanwhile I'm adding 6 DROPS of the remaining PMDD
components.  The plenum is working too well, it's making it harder to keep
things in balance.  Plants do great, then stall, depending on the nutrients.
When they stop growing, I measure, and sure enough, nitrates are gone.  Algae
is in there, but responds more to fish feeding than anything else.