Test the spring water before using it. A deionizer is going to be your most effective tool.
As far as greenery, at a PO4 level of 3.0ppm you're going to need a LOT of it. And also remember that the PO4 stays in the tank until you physically harvest the algae and remove it from the tank.
There's a good thread on building an algae scrubber. In your case I think it would be well worth the effort.