Beverly the thoughts are that nutrients (particularly phosphates) bind to the sand and then begin to release once it reaches a certain level.
FWIW alk and Ca are 3.0 meq and 400ppm respectively and NO3 is undetectable.
Steve what grain size are you using? Do you do maintenance on your sand bed or is it fill-and-forget?
I was thinking of mixing in some larger grain sand and start a vacuuming routine though I would be concerned about how that would affect critters.
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