In a 17,000 gallon pool, each quart of 6% bleach is only going to raise the cl by slightly under 1ppm. So a half gallon is only going to add a little under 2ppm. If he is only adding 1/2 gallon daily, that is not enough. With a cya of 40, the cl needs to stay between 3-6 ALL the time and the shock level is 15. So either he is going to need to use a larger daily dose of bleach or supplement with cal-hypo. I wouldn't suggest using triclor as his CYA is already 40. Don't really want it higher and the trichlor will increase it.

We really could use a complete set of current water testing numbers to better advise. Also what type of pool is this. It would be better to know these things first before recommending the use of cal-hypo. And, please remind us again -- are you in the pool industry or just helping a friend with his pool? (We have so many members and so many different threads that it is hard to remember.)

Thanks.

EDIT -- Since he has let his cl drop so low, I would suggest shocking it up to 15 just in case a bloom might be trying to get started since you can't always see it at first. Probably doesn't but it is cheap insurance just in case. Easier to do this than to clean up algae later.