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Phosphate/nitrate chemistry. A puzzle?

Dear list members:

I have a minor problem which I have been puzzling over for several months. I been keeping a 75 gallon and a 29 gallon tank, 
both heavily planted, and both containing fish for over a year. I regularly exchange with RO water supplemented with 1 tblsp 
sea salt and 0.5 tblsp sodium bicarbonate per 10 gallons, and use Kent iron supplement to maintain an iron concentration of 
about 0.1 ppm. I generally change about 30% of the water per week. What puzzles me is that  the 75 tends to accumulate 
moderate nitrate concentrations (10-20 ppm), but very low phosphate (<0.2 ppm), while the 29 does not accumulate nitrate 
(runs <5 ppm) but accumulates moderate phosphate (.2-.5 ppm). Other than the water changes and Kent plant supplement 
indicated above, neither of which contain either nitrate or phosphate, the only source of nitrogen or phosphate that I know I am 
adding is fish food.

 It has been my understanding from perusing the archives that fishfood tends to provide too much phosphate, and not enough 
nitrate. Ergo, the wide-spread use of PMDD (which contains nitrate), which according to the Conlin-Sears approach, should 
drive phosphate low by providing nitrate if nitrate is limiting. I confess I have not tried PMDD, and have used the Kent product 
instead, whose nutrient concentrations are not published. (I have measured the iron concentration, and have determined that it 
is somewhere about 2000 ppm.) Regardless, if phosphate is generally in excess in fish food, compared to nitrate, I can't figure 
out why both tanks don't accumulate excess phosphate. Does anyone have any ideas?

The tanks:

75 - (High nitrate, low phosphate tank): Eheim trickle filter with scintered rock only (no charcoal), plus a small power filter for 
additional circulation.
yeast-derived CO2; pH runs around 7; kH about 2, gH about 4 (from sea salt and sodium bicarbonate).
iron about 0.1 ppm; lighting 3.2 watts/gallon.

Fish load heavy: 12 juvenile angels, 18 cardinals, 4 siamensis, 4 1.5 inch "checkerboard ciclids, 8 Otos;
substrate: "Terralit" (a Tetra, baked clay material) underneath fine aquarium gravel). 

29 (low nitrate, high phosphate tank): external power filter with glass wool and sponge only (no resin or charcoal). 

yeast-derived CO2; pH runs around 7; kH about 2, gH about 4 (from sea salt and sodium bicarbonate).
iron about 0.1 ppm
substrate, as 75.
Fish load: two adult angelfish (about 3 inches long), one spotted Raphael (about 2 inches), and 2 Otos.
Lighting, 80 watts/29 gallons.

The fish food I use is Tetra Min flakes. I feed once a day, and the food disappears very quickly (within 2 minutes). I am adding 
considerably more food to the 75 than the 29, because of the number of fish. Yet this tank does not accumulate phosphate, 
while the 29 does. Where the heck is all the phosphate coming from??

rdenney at ibm_net
Richard M. Denney