how long I don't know. I just now that the nutrients to be used have to
removed from the substance. A heavily planted tank with lots of root
feeders would probably deplete it a lot faster than one stocked with mostly
stem plants. One to test might be with an iron test kit. Get a new sample
and crush it and mix it with some ro/di water and test the iron. then get a
sample of the old stuff and crush it and mix the same amounts of that and
ro/di water as you did with the other batch and test that. I don't know if
that will actually tell you anything about the usable iron or not, but it is
the only way I can think of to find out.

I would think all you need to do is start supplementing with iron if you
notice a deficiency. I don't think a complete teardown and startover is
needed. But hey, what the heck, sometimes it fun to do just that anyway.

--

Margolis
http://web.archive.org/web/200302152...qs/AGQ2FAQ.htm
http://www.unrealtower.org/faq