Phosphates/ Chemistry

Dirk  had a question about phosphates.
  Yes, water systems do add orthophosphate to control
corrosion in the water system.  Yes, the phosphate will
'tie-up' the iron fertilizer and make it useless.  The more
major problem from the phosphate is that algae loves
phosphate and will grow like crazy.  Even at the low levels
used by water systems of 2mg/l (ppm).
  Some water systems use Zinc Orthophosphate which will
increase the amount of zinc ions in the water.  Zinc is more
of a problem in salt water aquaria.