jdp39 said:
I am still curious also why the addition of phos remover has such a startling effect.
Because if you only have orthophosphate in the pool and not many organic phosphates then a phosphate remover will significantly slow down algae growth since they need phosphates to grow. So then the algae growth rate becomes much slower than chlorine's kill rate.
However, are you saying that before adding the phosphate remover you were maintaining shock levels of chlorine with the FC around 40% of the CYA level and the algae would continue to grow -- it wouldn't even turn from green to gray? Or are you saying that after lowering from shock level to an FC of around 7.5% of the CYA level that the algae would then come back? Yellow/mustard algae can come back at this level (it would need an FC that is around 15% of the CYA level to keep away) which is why one must kill it off completely.
I've had 3000+ ppb phosphates in my pool normally at 85ºF-90ºF temperature and though the pool was very reactive if I let the chlorine get too low, it was always algae free if the proper FC/CYA ratio was maintained. Others have similar experiences.
If you are using tabs, then your CYA is likely climbing and I suspect that your CYA reading is wrong from your test kit. You should really have a better test kit, either the TFTestkits TF-100 or the Taylor K-2006. If you are using
Aqua Chem 6-Way Test Strips then they are next to useless and I don't care what other pool stores or professionals you are comparing against since we've seen over and over again how they are often wrong as well. Even if you are using a drop-based test kit (such as the Aqua Chem Pro from Walmart), the chlorine test is likely still DPD comparing colors and is not as accurate as a FAS-DPD test kit (and I think in that test it's TC only and may have 50 ppm CH resolution compared to Taylor's 10 ppm). See
this post for a comparison of test strip vs. FAS-DPD drop-based test kit resolution. The accuracy is even worse for test strips than shown, especially for the CYA test (and test strips can't even test for CH). Even for DPD drop-based chlorine tests, some have found it to be less accurate than FAS-DPD (which can measure to within 0.2 ppm when using a 25 ml sample size).
You mentioned using Trichlor and your sig says you have an inline chlorinator (tabs), but for every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm. So even with a 2 ppm FC per day chlorine usage, Trichlor would increase CYA by over 35 ppm PER MONTH if there were no water dilution. If you find that your test kit is not having the CYA increase significantly over the months, then there's likely a problem with the that test. Without an accurate FAS-DPD chlorine measurement and an accurate melamine turbidimetric test for CYA, I don't think you can conclude anything about the FC/CYA ratio being the reason for not preventing algae growth.