Before a pool party I always bump my FC level up to 10 or 12 ppm. My CYA is 80, [FC/CYA][/FC/CYA]. Then I test again later that day or the next morning and usually bump it up again. Yesterday morning my FC was 12.5 ppm. 4 adults, my 17 yo son and his girlfriend and 2 OPKs swam in my pool for about 4 or 5 hours in the afternoon. SWG was running until 8p or so. This morning FC was 4, barely! The 2 OPKs were 10ish. What is an OPK? Other People's Kids. What do OPKs do? Pee in your pool. 
Chlorine is your friend!
Chlorine is your friend!