I am so glad I discovered the TFP site, and am grateful for all the knowledge contributed by the members. I have yet to see a book that explains everything I've learned about pool chemistry from this site. I knew that I was not getting a straight or consistent story from the pool stores, and for years I struggled with algae. I knew that high CYA was bad, and that the tri-chlor tabs I was using steadily increased the CYA. It was only at the TFP site that learned that I could stay out of trouble by keeping a proper FC/CYA ratio, and that I needed to get an FAS-DPD test kit to accurately measure the amount of FC needed. My local Leslies store does not even have these kits available in the store, so I had to order it online. One guy at the store even claimed I could avoid getting too much CYA in the pool by only putting a couple of 3 inch tabs at a time into my in-line chlorinator, instead of filling it full of tabs. Sigh!
I've had no algae problems since I started the following routine: I now use only bleach and muriatic acid for normal maintenance. Early each morning I check my chlorine with the FAS-DPD test, and if necessary I add enough bleach to bring it up to at least the target FC level in the "Chlorine/CYA chart", or at least 2 ppm above the minimum FC level, based on my current CYA level. I usually lose between 1.5 and 2 ppm of FC per day in the summer, so this generally keeps me at or above the minimum FC level after a full day of sunshine. I normally don't lose any FC overnight, so I check it again next morning, and I find it is always at or above the minimum FC level. The CC stays at zero, and I never need to shock it regularly (as the books and pool stores tell you to do). I don't need algaecide. After a day with a bunch of kids swimming for hours, my CC temporarily went up to about 1, but after a day or two it was back below 0.5, without shocking - just keeping the FC level at or above the recommended level. I keep a daily log of my measurements and the amounts of bleach or muriatic acid added, so I can anticipate my pool's needs. It's very easy and doesn't take much time, once you have a system. On a day when there has been a bunch of people in the pool, I will check the FC at the end of the day to make sure it didn't drop too low, but otherwise I only check it each morning.
When I am going to be away for a week or more, with no one to do the daily checking, I start using my in-line chlorinator with 3" tabs a day or two before I leave, so I can get it adjusted right. On my latest trip, I set the flow rate dial so that it would consume about one-half tablet per day*. In my pool, that should add 1.7 ppm of FC per day and 1.0 ppm of CYA (knowledge I gained on the TFP site, of course). My CYA was about 55 before I left, and I adjusted the FC up to about 9.5 in the morning. When I returned 7 days later, my morning FC was 10.5. About 3.5 tabs had been consumed during that time, so I figured that the chlorinator had added about 12 ppm of FC and 7 ppm of CYA during the 7 days I was gone. And thus the sun must have consumed about 11 ppm of FC during that time. I measured my new CYA level and it was around 60 to 65 as expected, so I will increase my FC level appropriately. I found that keeping a higher FC level does not seem to require me to use more bleach, since i only have to replace what the sun consumes each day.
If I take enough trips, with each trip requiring me to add 7 to 10 ppm of CYA (by using tabs), I know that eventually I will need to drain and replace the water, but at least now I know how to keep the proper FC levels and avoid algae, as my CYA gradually goes up. Thank you, Trouble Free Pools!
* My crude way of estimating the rate of tablet consumption is to measure the distance from the top tablet to the top of the feeder tube before and after a 24 hour interval. Since each tablet is 1.25" thick, if the level went down by 0.625", I know that it has dissolved half a tablet per day, and for my pool it has added 1.7 ppm of FC.