Clarifiers only help the filter filter by making the particles clump together -- if the filter sux, clarifiers don't help much. Besides that, clarifiers can only make certain types of particles clump -- I'm not certain, but I don't think the phosphate particles are the 'right' kind.
Ironically, if the particles aren't the 'right' kind, clarifiers actually make things worse!
Regarding the corrosion -- the "corrosion" is probably just normal 'inside of the iron pipe' corrosion; the water company wouldn't consider it anything that needed fixing. In any case, the 'fix' is to replace the water mains entirely, and in this economy, that will NOT happen till it has to happen!
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