phosphate diffusion from substrate

Karen's observations about increased phosphate levels after putting
DAI tablets in the substrate prompted me to purchase a phosphate
test kit last night. I got the Aquarium Pharmaceutical dry tab test
kit (any comments?). I'd have preferred a LaMotte one but haven't been
able to locate one here in Vancouver. Anybody seen one?

I did the test on both aquariums and found both to have about 1ppm
of total phosphates, perhaps a little less since the blue color
appears to be caused by a fine suspension of particles rather than
a dissolved indicator color. Anyway, shook it vigorously and then
squinted to get an average color. Both of these aquariums have
plant stix fertilizer pushed as deeply into the substrate as I
can manage and in place for a couple of months. There doesn't 
_seem_ to be a problem in my tanks with phosphate diffusion
or leaching from the substrate. I use Salvinia; perhaps it could
reduce phosphate concentrations somewhat... I also fertilize
twice weekly with Flourish so perhaps the Fe helps precipitate
excess phosphates too.

One problem with some fertilizer tablets might be that they
dissolve or become soft almost immediately upon getting wet. When
one pushes them into the substrate, a fair bit of the soft part
could break off and mix into the water or the surface of the
substrate. Something to think about.  I wonder if that could 
have been a problem with the DAI tablets Karen? A wise precaution
is probably to put any fertilizer tablets into the substrate
only during the tank set-up until it's absolutely necessary to add
more if/when there appears to be a shortage of nutrients. Another
precaution is probably several large water changes every few
days if phosphates test high after adding substrate tablets or
sticks. I will surely check if or when I add any more. As some
folks have said, it's probably unnecessary to add nitrate or
phosphate fertilizer if the tank is well populated with fish
and you are feeding regularly. The ones I added appear to have
made little difference but it's so subjective. On the other
hand, more frequent Fe dosing and water changes have definitely
improved the Rotala and other difficult plants.

Thanks to all who've commented on the anaerobic topic; it's been
quite informative. Now we know more good reasons to maintain
adequate Fe dosing and to maintain high iron levels in the 
substrate itself! (H2S preventative) and dispell some of the
fear-uncertainty-doubt about anaerobic substrates and sulphates.

Steve              Vancouver BC.