Losing just a little FC overnight

I had some problems with FC staying at 1, so last night I used enough bleach to get it to 18ppm. I tested at about 7:30pm and it was 18ppm. I tested again at 7am this morning before the sun came up and it was 15ppm. So it did not lose a lot, although it was more than 1ppm that it lost. So in this case do I need to just bring it back up more tonight?

My pool is around 21,000 and 15ppm was shock level so I guess it was still technically at shock level this morning. I am not sure if I need to add more bleach being it did lose 3ppm overnight and not just < 1ppm.

Thanks everyone!!

Since you lost 3ppm overnight, I'd go ahead and shock it up again tonight after you test it. Then, a few hours after adding more bleach, test again so you'll have a number to compare it to in the morning to look for overnight drop.