FC loss scenarios


Well-known member
Jun 19, 2014
Spotswood, NJ
Hello All,

Had a couple questions regarding FC loss

1)Got (over) thinking about something I have been wondering for a while:
On a normal day, I would dose the pool to get it to 6ppm FC and 24 hours later it was usually around 3FC, which I believe to be in a normal/accepted loss. (lost 3ppm or 50%). Assuming everything else is equal, does that mean if I had instead dosed the pool to 10ppm (I know that is too high, that number is just for this example), I would have see it drop the same -3 to 7 or drop the same 50% to 5? I am trying to understand is FC loss is a solid number (again assuming everything is good and no contaminants, etc) or a % ratio. Shorter way to ask: Will a higher FC drop faster when you start at a higher number because there is more to lose or at the same rate as a lower starting point. Boy, I hope that makes sense.

2) Ok, next-as I understand it the more sun (UV) on a day, the more it will eat the chlorine-so should i expect on a cloudy day I would lose less and on a sunny day I would lose more?

3) Lastly-will a solar cover help slow FC loss due to sunlight because the light isn't actually hitting the water and is instead contacting the cover which is passing the heat to the water?

Separate concern, but might as well ask while I am posting: I am SLAMing for what a few here have identified as dead algae on the bottom of my pool (poofs away when I try to touch it and only shows up only shows up in the vinyl crevices when the pump is running). Had this stuff for a LONG time. When I am finally rid of the issue according to SLAM rules, how do I get this stuff out of the pool? I assume the second I touch it with the vacuum, it disintegrates before most of it gets sucked up, so it will just swirl and get caught in the crevices again. Or do I brush it and let it swirl so the skimmer gets it? Picture below to show what it is as it sits now



Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
Tucson, AZ
I would expect you to lose approximately the same percentage of your chlorine to the Sun regardless of your starting level. So if you start at 10:50 p.m. I would expect you to lose 5 PPM. Less Sun would also mean less losses and a cover would also lower your losses to the Sun.