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?
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.


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


Thanks! I've tried adding potassium nitrate, getting the level as high
as 10 ppm on one occasion. Normally nitrate is close to zero.

Some of the rainbowfish appear stressed for the next day or two after a
nitrate addition, and I only add a little at a time. Should I very
slowly ramp up the nitrate over a month, using potassium nitrate
solution? Do you or others find that this stresses the fish? I have:

2 small pl*cos
2 siamese algae eaters
4 clown loaches
4 neon dwarf rainbowfish
1 large blueish rainbowfish
12 zebra danios
15 white cloud mountain fish
9 platies

Alkalinity in the tank is about 80 ppm, because of crushed coral used in
filter (tap water is very low alkalinity).

Based on your suggestion I will try to very slowly bring nitrate up to
30 ppm and see what happens. If the fish get stressed I'll do water
changes to bring nitrate down again. Any further comments are welcome,
thanks!
Steve