Solar cover? Probably off -- higher water temps = greater chance of algae.

BUT the solar cover also reduces chlorine loss to sunlight. So, I guess it would go this way:

1. If you try to 'SHOCK' (high chlorine) your way thru the vacation, use the cover

BUT

2. If you use polyquat, per above, then leave it off. Polyquat is NOT lost to sun, and the lower temps will reduce the chance of algae.