I didn't state that the sand change was going to get rid of the green pool. The reason the pressure on a sand filter increases is due to dirty sand.
Actually, old sand works pretty good for most of us with sand filters. The psi increases due to your filter cleaning algae and gunk from your pool water. This is NORMAL. When your psi increases about 6 over your clean psi reading, it is time to backwash.
If you are having to backwash a filter once a week that is too much.
That depends on WHAT the OP is trying to filter/clear up! This spring, I had to backwash TWICE a day to clean up gunk and my pool was not green!
When your sand is that old it can take longer to clean the pool up.
We have some members with sand really, really old (up to 20 yesrs old) and have no plans to change it.
I do not disagree with the fact that chlorine is going to clear the pool up, but in order for you to get the most effectiveness from your shocks, you need to have a good pH level as well as a cyanuric level of at least 30ppm.
"At least 30" is for those that run on unstabilized chlorine. Recommended levels are listed in Pool School. It all depends on your method of chlorination.
I deal with pools everyday and I usually can clear a green pool up within 48hrs.
Here, I will simply question the "side effects" of your 48 hour cure. There are NO magic bullets! Most of the BBB users here rarely have a need to shock their pool(s).....unless they are doing it for a specific reason, such as vacation, winterizing, fecal accident, etc.
If you remove high phospahtes then algae won't have anything to eat to grow. Natural Chemistry have great phosphate products.
We deal with swamps on a daily basis, and I am guessing more than you do. Phosphates are irrelevent in almost ALL cases of clearing a pool. Phosphate removers are just a scare tactic from pool stores to sell more unnecessary products.
Please spend some time in Pool School and post any questions you may have in the Deep End for discussion.