threewide said:
Once algae starts it produces it's own phosphates, which is why during an algae outbreak your Phos. count is higher.
Just for clarity, this isn't exactly what happens. Though dead algae that gets broken down can then release phosphates that were built into various organics, it isn't true that algae that grows produces it's own phosphates. Phosphates don't just spontaneously exist. What can happen, however, is that if the chlorine gets close to zero then bacteria can grow and they can efficiently convert any organic phosphates in the water into the inorganic phosphate (orthophosphate) that is what the phosphate test kits measure.
Algae can also utilize organic phosphates, but its uptake is slower compared to orthophosphate. This is why one can measure near zero phosphate levels on the test kit and yet still get algae. It's because of organic phosphates that are in the water.
Nevertheless, it is generally true that pools with high phosphate levels AND sufficient nitrate levels and sunlight and temperature can be very "reactive" in that the algae can grow near its limit (based on temperature and sunlight) which is roughly doubling in population every 3 to 8 hours. I am one of those who has had 3000+ ppb phosphates in my pool in the past and can say from experience that IF one lets the chlorine get too low for more than a day, then the pool can seemingly turn rather quickly in just a few days.
So while we all say here that chlorine alone can prevent algae growth regardless of phosphate (and nitrate) levels and that this is technically true, IF you don't believe you can consistently maintain a chlorine level then you have several options you should look at. One is to automate your chlorine dosing via The Liquidator or a peristaltic pump or using a saltwater chlorine generator (options that teapot mentioned). Another is to spend more money on supplemental products as "insurance" against algae growth. The two "insurance" approaches with the least side effects are to use PolyQuat 60 weekly or to have 50 ppm Borates in the water (or both if you want extra protection). These are not foolproof, but they will slow down algae growth probably long enough for you to deal with any issues. The next best choice would be to use a phosphate remover, but as I noted above it may not work well if you have a lot of organic phosphates, but should be seen in the same vein as using a weekly algaecide though can get very expensive if you already have high phosphate levels. The other options aren't so good, like using a copper-based algaecide, since this can lead to staining unless you are very careful with dosages and pH control.