OK. After I posted above suggesting that you use Polyquat 60% algaecide, I started thinking -- filtering will just remove the polyquat. So, I talked to Ben about your situation, and here is his recommendation for you:


I would use polyquat to maintain an unopened pool in summer. But the process I'd follow is this:

1. Hold the chlorine above 10 (if vinyl, not too far above!) for a few days with the cover partially off. After the first day or two brush the pool to make sure you have exposed any algae.
2. Let the chlorine drop to 3 and replace the pool cover.
3. Let the chlorine continue to drop to 1, and then begin adding weekly maintenance doses of of polyquat.
4. If there's a pump timer, operate for 2 hours, 2x per day.
5. Both filtration and chlorine remove polyquat. So low chlorine and low filtration extend polyquat's life.
6. If clarity drops -- use a waterproof flashlight at night -- filter full for 24 hours.
7. If there's a slimy feel to the vinyl walls, or any sign of algae, restart the process.

There are other ways, but that should work.