Trace elements and tap water

It seems that balancing trace elements can be a tricky business.
I have read that too much of one element can interfere with a plant's
ability to take up another. I know that tap water usually has higher 
concentrations of dissolved metals than is found in nature. I also
know that trace elements are usable by plants only in certain forms 
(chelated). All these facts lead to a confusing situation: even if I 
get an iron test, how do I know what part of the iron is in usable 
form? Does an excess of unchelated iron cause problems with other 
trace elements? What is the best way to remove excess elements (ie 
iron and copper) from tap water? 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. I tend towards red
algae problems even with regular (weekly 20%) water changes. I use
dupla drops, good lighting, undergravel heater, etc. and my nitrates
are barely registering. Just an idle thought: does the red pigment
in red algae indicate some affinity between it and high iron content?
Anyway, my highschool chemistry hasn't equipped me to deal with these 
issues, and I can't believe that dupla drops or any other trace 
element additive will fit all situations equally well.

Glen Osterhout