Weger:
Welcome to the forum and please add your signature. You state that the walls go green fast. Is that because upon closing, there is so much sunlight coming through the mesh cover that the chlorine burns off quickly? As Marty mentioned, if you are testing properly with a quality test kit and following the recommended FC/CYA chart, then you have a few options.
1) If you want to close with high chlorine levels, then no algaecide should be used.
2) If you want to close with normal levels, then you can add 1.5 quarts of Kemtek PolyQuat 60 algaecide where you can purchase on Amazon. This is the brand that seems to be used time after time.
3) Lastly, you can take a very thin large tarp or many smaller tarps (purchased at Wal-Mart) and cut a few slits in them and lay them over the existing mesh cover. These will have to be secured with string, etc. This way, water will seep through the cover and sunlight will be avoided as much as possible. Cover at least 80% of the cover, but no more than 90% of the cover. This method works and has been used before.
Most important, close late, which appears that this is what you are doing and open up early in the spring. At least one month prior to the holiday season. The pump run time and chemical consumption will be at a minimum. If the pool is not properly balanced, then this should be taken care of first and foremost.
