Trace elements and tap water

From: glen at harlie_pps.com (Glen Osterhout)
> I was told by someone at a fish store that you can remove dissolved
> metals with a polyfilter, and that you can tell what is being removed
> by looking at the color of the filter after it runs for a while (green
> => copper, red => iron) but I don't know how reliable this advice is.
> My polyfilter does show a definite reddish tinge after a day or so.
The Poly Filter by PolyBio Marine will most certainly remove virtually
all benefitial trace elements from a planted tank.  It's a fantastic
filter for removing heavy metals and other wonderful things from your
water.  PolyBio Marine makes excellent water purification filters and
the fish store certainly didn't mislead you.  However, I doubt that it
is good idea to use this filter, in-line, on a planted tank.  If you are
relying only on the trace elements found in your tap water, the poly
filter is most likely removing all "trace" of them (pardon the pun).
This type of filter is great for pre-filtering your tank water before a
water change though.  I'm considering using one of these filters as a
prefilter for my RO/DI unit.  I'm hoping it will help extend the life of
the Deionization filter.