Bleach

[CarlD]

High FC can cause false high pH readings. I don't know if it also can generate a false high TA.

Meanwhile, when you say you adjust TA, are you doing it by our ratchet-down, aeration method? You lower pH with acid to about 7.0. Then aerate to raise pH without raising TA. Repeat when pH is back around 7.5 until TA is where you want it.

If you aren't doing it this way, you aren't lowering TA...so I'm wondering if that's it.

Carl

[aylad]

And just to add one more comment--shock level for a CYA of 100 is 25 ppm. The normal, baseline, everyday chlorine levels for a CYA that high is 8-15 ppm. If you've been keeping your chlorine below 8 ppm, that's why you keep getting algae.

See the "best guess" chart linked in Watermom's sig for more info....

Janet