Pool stores don't know what POLYQUAT is...

[KirstenHW]

Is Polydimethyldiallyammonium Chloride bad?

[chem geek]

Is Polydimethyldiallyammonium Chloride bad?

No, it is not bad, but it is something "extra" you are adding to your pool that may or may not help. The "claim" is that whereas PolyQuat (the long name in the previous post) works best to prevent green algae, the additional poly component helps to prevent black and yellow/mustard algae. This claim is not borne out by experience on this board (or by Ben's experience) which is why we generally only promote use of PolyQuat. That doesn't mean it doesn't work -- it just means we don't see strong evidence for it. It's in GLB Algimycin 2000 which is a product I used to use and I didn't find any problems with it, but remember that I got my pool chemicals for free from a friend in the industry (I'm sure that'll stop after everything I've posted in this board!). [EDIT] This extra substance is a cationic polymer used as a clarifier. I suppose that for algae, this might help make the dead algae "clump" so that it is more easily filtered. This normally wouldn't be necessary because high chlorine will breakdown the smaller suspended particles while the larger ones should get filtered out by a good filter. I'm not sure where the "claim" of this product being for more types of algae comes from except that possibly the product decomposes (probably in sunlight) to an ammonia byproduct and that will combine with chlorine to form monochloramine that can be effective against some algae (but only if it gets to the algae before further breaking down to nitrogen gas). [END-EDIT]

Some algaecides contain copper to combat black algae and some contain sodium bromide sometimes with sodium tetraborate pentahydrate (similar to Borax but containing less bound water) to combat yellow/mustard algae. The experience on this forum is that these are not necessary, neither for prevention nor for killing algae.

What we HAVE seen on this board is that yellow/mustard algae can be quite nasty, come on suddenly (usually around August) and require higher chlorine amounts to shock than what is in Ben's table (apparently needs around 1.0 ppm disinfecting chlorine, HOCl, compared to 0.3 ppm for most other algae). We do not yet know what level of maintained chlorine is necessary to prevent such outbreaks, but it is likely higher than the low-end of Ben's table. I suspect that the use of Borates (Borax -- 50 ppm Boron) might work to prevent these outbreaks, but we do not know (yet) if this is true.

Experience on this board shows that black algae seems to be effectively removed by high chlorine PLUS scraping off the heads with a wire brush and rubbing Tri-Chlor tablets on the exposed algae.

This is my understanding of what we know, but of course with more experience we may find even better ways of controlling and removing algae while not just throwing chemicals in the pool that either don't work or that somewhat work but have serious side effects.

Richard