3des said:
Is there ever a level where the scale tips based on the amount of phosphate in the pool? For example, would the recommended FC level ever be consistently higher just because there is more phosphate than what is considered normal? Or should keeping the water balanced based on normal recommendations keep the phosphate in check no matter what its concentration is?
The short answer is no. Phosphate is not the only limiting factor to algae growth. Ultimately, the rate of growth is also limited by sunlight and temperature, both of which have optimum levels which allow for maximum rates of growth for particular species of algae. Giving the algae more phosphates than they need for that rate of growth does nothing. Even under ideal conditions, algae only double in population every 3-8 hours, depending on species.
Now, that said, a pool rich in algae nutrients (phosphates and nitrates) with warm water and ideal amounts of sunlight can be very "reactive" in that letting the FC get too low for too long has the algae grow seemingly quickly, but as noted with the growth rates it really isn't that fast. It's just that with the FC too low for an extended period of time or near zero for a day, then with 3 hour generation time algae can increase in number by a factor of 256 in 24 hours. Usually it takes a couple of days before this becomes noticeable, either as increased chlorine demand or as cloudy water. Of course, keeping the appropriate FC/CYA ratio keeps the chlorine killing algae faster than it can reproduce. The
Chlorine / CYA Chart is designed to prevent algae growth regardless of nutrient level for most algae (green, black) and other organisms and pathogens (white water mold, bacteria, viruses, protozoa except for
Cryptosporidium parvum oocysts) except for yellow/mustard algae which needs to be eliminated explicitly (or a higher maintenance FC/CYA level is needed, but most people choose to completely eliminate the algae instead).
Note also that the organic phosphates that the phosphate removers do not remove are only slowly taken up by algae, but are rapidly taken up by bacteria and converted to the inorganic phosphate that algae can use more rapidly. If the FC level gets close to zero such that bacteria can grow, then you can rapidly convert one phosphate form into another so you can go from a situation of apparent control to one that seems out of control fairly quickly and likewise phosphate tests can go from apparently low phosphate levels to high ones.
As Jason notes, it is just so much simpler to not worry about phosphates and instead maintain the appropriate FC/CYA level. In any event, phosphate removers should be seen in the same vein as algicides -- something that isn't necessary, but is a supplement you can use at extra cost if for some reason you are not able or willing to maintain the appropriate FC/CYA level. If one wants a supplement (which again, isn't really necessary), we normally recommend using Polyquat 60 weekly since it has fewer side effects or one can use 50 ppm Borates since they are also a mild algicide.