Most pool stores can do a water test to discover if there is a problem with the water. I use a cartridge filter on my 18' 54" deep above ground pool. I really like the cartridge filter but I have not used a sand filter. I don't have to back-wash, just pull the cartridge out and hose it off. I have heard of someone with high copper content in the water causing the liner to green. I use the flocculant(clarifier) and algecide from lowes, but always use high quality 3" chlorine tabs from a pool store. I also shock with a cup or so of 15% sodium hypochlorite(bleach) but I am not sure if that is available from a pool store. I use a gallon sodium hypochlorite and a bottle of algecide to winterize and the water is always clean when I uncover it.
when you are about vaccum fill the water up a little more in the pool and then put the filter on waste and vaccum the worst stuff right out of the pool, this was how we did it every year as a kid when we opened the pool . Make sure you use a copper based algacide. cost more but works