purple algae again

[pool girl]

Me again, actually it test high for "Cyanuric acid" What is this and how do we lower it. As we are told to drain the pool.

[chem geek]

Unfortunately, you can only readily lower Cyanuric Acid (CYA) by dilution, so a partial drain/refill. You can learn more about how to maintain your pool by reading up at the Pool School. To prevent algae in the future, you need to either maintain a sufficiently high Free Chlorine (FC) relative to the Cyanuric Acid (CYA) level -- for a manually dosed pool that means an FC that is at least 7.5% of the CYA level at all times. Either that or if you want to use a lower FC level or not care about the CYA for as long then you need to use a supplemental algaecide (e.g. PolyQuat 60) or a phosphate remover at extra cost. Or you can regularly dilute the water through more frequent or longer backwashing (if appropriate for your filter) or intentional dilution (regular partial drain/refill) to keep the CYA level from building up.

Chemical facts that give you an idea of how your CYA level could have built up quickly are the following:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.

Richard