Hi, new member. 16500 gal. white plaster inground pool.

FC 12 - 20
pH 7.8
T/A 90
CH 220
CYA 75

So. california area where we get a lot of sun. I am keeping my minimum FC level at or above 12 after battling mustard algea. The water is as clear as I've ever seen it and everything is stable but I'm using a lot of Chlorine. I buy it at Walmart (the 10% stuff). After a reading of 13 or 12.5, I will add to around 20 to bring it up. In just 2 days I'm already back to around 12 again. I read where the FC usage will or should fall to around 1ppm per 24hr period. Am I going to stay at 4ppm usage per day because of the CYA level or am I missing something?

You don't lose a fixed amount, you lose a percentage. So the higher you maintain FC, the more you'll lose.

In simplified terms, with CYA 75, you really only need to maintain about 9 minimum FC. The rest is unbound to the CYA and at the mercy of the sun.

I'm sure Chemgeek will be along to explain it better.

Thanks for answering. So, I would be better off checking it every day and keeping the FC level as close to 9 as I can without falling below that number? I was kinda trying to give my self a day off (so to speak) Thats why I would build it up to 20 or so. Oh, the 12% is the Mustard Algae Minimum, unless I can safely go back to regular dosages now that the pool is clean. Don't want to give the Algea another chance to get a foot hold. Anyway, to maintain 9, and the pool using 3% a day, would it be safe to increase to 18 so as to not have to test every day. Or would that just be waisting Chlorine to the sun.

At any given CYA level the higher the FC level the proportionately higher chlorine loss. So with 75 ppm CYA at 9 ppm FC you might lose 2 ppm FC but if you were to raise the FC level to 20 ppm then you'd lose more than twice as much so more than 4 ppm FC. Adding chlorine more frequently, such as every day, loses the least amount of chlorine though is obviously more work.

There are various ways to reduce the amount of chlorine usage, though 2 ppm FC per day loss is pretty typical with most losing from 1.5 to 3 ppm per day depending on sunlight exposure and CYA level. Some people get automated dosing systems such as a peristaltic pump or The Liquidator while some get a saltwater chlorine generator system. If you have a mostly opaque pool cover, then that cuts down chlorine loss. That's the situation for my pool that is open 1-2 hours every day and has a little less than 1 ppm FC per day chlorine usage at relatively warm 88ºF pool temperature.

If the algae is all gone, you no longer need to maintain FC at Mustard Shock level.

How do you know?
1) The pool is clear
2) CC is no more than .5
3) You lose no more than 1 ppm FC overnight.

Would that mean that 2 or 3 ppm FC loss overnight might suggest that there is still algae present? I'll bring it down to 9+ and watch the fall off for a while. That's the only one of the three conditions I haven't met yet. Thank you for your help.

It means algae, organics or something else that needs to be oxidized.

Yes. Any loss of 1ppm or more overnight means that there are still organics present. Keep shocking.