As that link stated, "Preventing algae from regrouping requires that either the chlorine concentration is kept at high levels or that the conditions in the water are changed to be less favorable for algae growth." This is true, except that "high levels" for chlorine does NOT mean a particularly high active chlorine level. It takes only a hypochlorous acid level of around 0.03 ppm to kill green and black algae faster than they can reproduce regardless of nutrient level. Yes, you read that right. So why is it that pools get algae even though they have chlorine in them? The answer is Cyanuric Acid (CYA) aka stabilizer or conditioner.
CYA significantly lowers the active chlorine (hypochlorous acid) level in pools. When the Free Chlorine (FC) level is around 7.5% of the CYA level, over 97% of the FC is bound to CYA and does not kill algae, disinfect, nor oxidize to any great extent. This is known science since at least 1974 (with
this paper). At this FC/CYA ratio, there is only 0.03 ppm hypochlorous acid and 0.03 ppm hypochlorite ion (at pH 7.5). The reason that many residential pools get algae is that most use Trichlor pucks/tabs where the CYA level increases over time while people do not increase their FC target over time (i.e. the FC/CYA ratio drops over time). The following are chemical facts that aren't taught even in your Certified Pool Operator's (CPO) training course:
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 at least 7 ppm.
At 2 ppm FC per day, Trichlor increases CYA by over 35 ppm per month if there is no water dilution. I strongly suggest you read
Certified Pool Operator (CPO) training -- What is not taught if you want to know more.
My own pool has had over 3000 ppb phosphates and there are other pools with over 5000 ppb phosphates (and lots of nitrates as well) that have no algae growth because the proper FC/CYA ratio is maintained. There is no question that such pools are more "reactive" if you let the FC/CYA ratio get too low since the algae do grow more quickly with the higher levels of nutrients, but there is a limit to such growth based on sunlight and temperature where algae double in population in 3-8 hours under ideal conditions. If you've killed off the algae and maintain the proper FC/CYA ratio, then chlorine kills any new algae getting into the pool faster than it can reproduce.
The current recommendations by many saltwater chlorine generator (SWCG) manufacturers of 1-3 ppm FC with 60-80 ppm CYA is NOT sufficient to prevent algae growth which is why you see problems in some of these pools. It doesn't even show up as algae initially, but rather as a mysterious chlorine demand where one can't seem to maintain or get a chlorine level. There is no mystery here -- nascent algae growth consumes the chlorine as fast as it is reproduced IF you let things go too far so that there is enough algae present to create that amount of consumption. For SWCG pools, the minimum FC is 5% of the CYA level so 4 ppm FC with 80 ppm CYA would work, but only if you got ahead of the algae in the first place (i.e. shocked the pool to kill off the algae, if it is already present).
There are tens of thousands of pool owners at
The PoolForum and
Trouble Free Pool who maintain their pools according to either the
Best Guess Swimming Pool Chlorine Chart or the
Chlorine / CYA Chart. Most of these pool owners (including myself) use only chlorinating liquid or bleach (or have SWCG systems) and do not use any algaecides, phosphate removers, clarifiers, flocculants, enzymes or even shock regularly. We maintain our pools based on known science, not on what the industry says in order to promote highly profitable products.
Now if you are a service tech and only visit a pool once a week, then that's a different story. Though there are some in sunny desert areas that use high 100 ppm CYA levels and use chlorinating liquid (or chlorine gas) to raise the chlorine to 14 ppm which then drops to around 4 ppm the following week when more chlorine is added, most use Trichlor tabs/pucks for continual dosing. Since the CYA builds up making chlorine less effective, they need to do something to prevent algae growth so add algaecides such as Polyquat, or they add borates (Proteam Supreme, Optimizer, etc.), copper ions, or use phosphate removers. These are all extra cost, of course. My local pool store runs a pool service for around 2000 pools and they try and maintain 4.5 ppm FC and dilute the water if the CYA gets above 100 ppm. Even so, a small number of pools get algae and need shocking or algaecide or phosphate remover. Again, not a surprise since their target FC is a little too low for the maximum CYA level they allow.
In some areas, the CYA buildup is not a problem because the pool seasons are short, the pools are smaller, sand filters are used so have regular backwashing, and there is summer rain overflow so CYA doesn't get high enough to be an issue and winter rain overflow dilutes the CYA even further for the next season. However, other areas have no summer rains, use over-sized cartridge filters with infrequent cleaning, have larger pools, and longer swim seasons so the CYA builds up more quickly.
I'm not saying that phosphate removers don't work, but rather that they should be seen in the same vein as algaecides. They are something that is not necessary if you maintain the proper FC/CYA ratio, but can be used as insurance if you are unable or unwilling to do so.