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I'm a huge fan of the wonderful substrate work you published in PAM. It's
just wonderful to have a real scientist on board working substrate issues.
I saw your note on the APD to Chris re nitrates and nutrients. I have been
surprised by how much nitrate a well-run planted tank can tolerate. An
interesting data point is included in the original Dupla book, The Optimum
Aquarium. Somewhere toward the end of the book the authors mention that the
tap water they start with at Dupla contains around 54 ppm of nitrate and the
water column in their large tank on average runs around 25 ppm. On two
occasions I have seen tanks that had over 100 ppm of nitrate that were not
overrun with algae. Of course no one would ever recommend this level, but
it just might be possible to work with water with 90 ppm of nitrate.
Perhaps counterintuitively, my guess is that a 'balance' for such a
situation might include adding a fair amount of K and almost certainly PO4
and micronutrient supplements. The idea would be to drive the nitrates down
by ensuring adequate supplies of all other nutrients.
You mention having spot algae with 5 - 10 ppm nitrates and 0 ppm PO4.
Several of us here on the West Coast have observed that well-run high light,
CO2 supplemented tanks often show a very noticeable benefit from the
addition of small amounts (under 1 ppm) PO4. This can on occasion include
reduction of spot and other types of algae as well (anecdotal observations
only). If your plants are short of P, you will see an immediate affect from
adding P. Claus Christiansen described the following pattern to me. One
adds a small amount of PO4 and within a few hours the plants pull the
measurable level down to zero. The plants have an obvious vitality which
lasts for a few days at which time the system will benefit from another
small addition of PO4. During most of this time the water column will not
show any measurable amount of PO4, but the plants seem to benefit from the
additional P. Recall that Karen's new water supply contains something just
under 1 ppm of PO4, if memory serves, and she has not noticed any algae
problems. My current guess is that Marin County's 'Magic Water' for plant
growing is due in part to small levels of PO4 in the water supply.
When we get together out here one of our recurring themes is the possibility
that the Sears/Conklin conclusion should be reformulated as follows: We
reduce algae not by limiting P, but by assuring that N does not become the
limiting factor. And we often note that some systems appear to benefit from
the addition of small amounts of P every few days.
Regards, Steve Dixon