...the 1-3 ppm FC is more than adequate to control the water-borne bacteria...
Understand that this is 1-3ppm
without CYA.
Cyanuric Acid, while it helps to buffer available chlorine and hold it in reserve against UV degradation, also reduces the 'effective strength' of chlorine. It's a trade-off, longevity for strength. It's accepted here because of years of observing it in action. Much of the pool industry information tends to just carryover from water treatment science, which is different on a variety of fronts, not the least of which is that their chlorination is not stabilized against UV, and their water is not recirculated.
The 'industry' seems to like it this way. They can start you in your nice new pool with a car-trunkful of chemicals. Stabilizer, tabs, broadcast powder, "shock", ph-up, TA-down, more PH-up, calcium, algacide, and more. You have a nice new pool with no idea what's going on, but it's evidently working. Expensive, but working. Each trip to the pool store will net you a wonderful free water analysis by a kid who is undoubtedly a highly trained water purification professional, and a cartload of fresh chemicals. But hey, pools are expensive, right? Then, as the stabilizer begins to build up and the chlorine becomes less effective, they'll tell you to shock with their special high-potency powder. Oh yeah, and some ph-up. And clarifier for the cloudiness. The price you have to pay for having a pool to enjoy. It's worth the cost, though, right? And that'll work periodically, until the CYA level gets too high to even get to an effective shock level. Then they will tell you that you have to eliminate the algae food with phosphate remover. It's expensive, but you know the drill... if you want to have that pool, you have to take care of it. Oh, and more of that shock powder. And some TA-down. Next time in, you'll get some ph-up and more clarifier. And some algaecide. And some stuff to reduce foaming. You'll bounce back and forth between TA and PH adjustments, and keep trying to kill algae. The guys at the store will ask you lots of pointed questions, like "Are you sure you're adding it like we told you to?" and "Did you get collect the water sample properly?", which will make you start to doubt your ability, and they will send you home with various expensive products until they tell you that you have a condition known as 'chlorine lock', which makes your chlorine no longer effective, and they'll have you partially drain and refill your pool. Then you'll need some more tabs and powder for the new water, along with some algaecide and various containers of ph and TA adjusters. You'll probably be OK for a while, depending on when and how much water you drained out, but it all repeats until you learn that you have developed 'chlorine-resistant' algae, and will need to do another drain/refill and use much more of their chemicals this time to get your water back to OK.
Or, you can just follow the CYA/chlorine recommendations, use a little borax or baking soda when needed, and not ever have to deal with any of that experience or expense.
Totally your call.
