Richard Sexton wrote:
In article ,
Steve wrote:

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?


Add nitrate. BGA cannot grow in the presense of enough nitrate (30 ppm).


Richard Sexton's suggestion of adding significant nitrate appears to be
working. This weekend nitrate was up to about 15 ppm and the BGA seemed
to be starting to "melt". In any case it's not growing rapidly around
floating plants like it used to. I've added more nitrate with K and
trace elements, and nitrate should now be around 20 ppm. with more
additions to come.

Following up on Richard Sexton's suggestion by google searches, I found
that 10:1 N:P should inhibit BGA. Here's a reference:
http://www.nalms.org/lakeline/pdf/ll21-1_smith.pdf . 20 to 30 ppm
nitrate should give me N:P 10, considering the amount of phosphate
reported by the city.

Thanks Richard!
Steve