polyphosphate blue-green algae

I finally looked up some city water works reports on the Internet and
they show variously:

- total phosphates up to 0.74 mg/L
- total phosphorous 0.17 to 0.22 mg/L (different time period)

- alkalinity 29 to 31 mg/L

Ever since the later '90s I've had blue-green algae (BGA) problems,
although they can be kept within reasonable bounds by manual removal at
water change time. The start of BGA seems to correspond to about the
time the city started adding polyphosphate to the water as a corrosion
inhibitor.

Do people have simple suggestions of how to reduce or moderate the BGA?
Folks at work are suggesting a reverse osmosis system with subsequent
remineralization of the water, but I'd like to keep it simple.

- 90 gal Aquarium
- 160 or 200 watts fluorescent light
- Eco-Complete substrate
- many plant (crypts, vals, Rotala..)
- good stock of fish

No fertilizers are currently being added to the aquarium, although I've
tried potassium nitrate solution, various trace element solutions,
Jungle plant tabs and even Pond Tabs. No C02 system is used.

Thanks for all suggestions!

Steve