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:

I once screwed up and put 200ppm of nitrate in a tank. I took me a week
to notice and about 3-4 weeks for the plants to eat it all up. No fish
snail or any of the shrimp in that tank was affected. I keep my tanks at
30-45 ppm nitrate.

Ammonia stresses fish, not nitrate.

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