Lime pellets won't have that effect, but even a small amount of fertilizer would use up a great deal of chlorine.
It is also possible that there was a hotter than average day and FC fell and algae got started. Your CYA level is very low for use with a SWG, which can result is rather large swings in the FC level. Some algae might simply have taken advantage of a larger than usual swing in FC level.
It is also possible that there was a hotter than average day and FC fell and algae got started. Your CYA level is very low for use with a SWG, which can result is rather large swings in the FC level. Some algae might simply have taken advantage of a larger than usual swing in FC level.