I have an above ground pool with a sand filter. My filter cracked and I replaced it with a larger filter due to a price I couldn't pass up on. I notice that after about an increase of 5 psi (from 10 - 15 PSI) the return flow is very weak and I have to backwash. It can't get pass an increase of 5 PSI. I am thinking that my pump is under-powered for the filter. I was told by the seller that the 1.0 HP pump I have would be fine but I think I may need to upgrade to the 1.5. Am I on the right track or could it be something else? I have checked for obstructions including in the impeller and there are none. I just wanted to get some other opinions before I go buy another pump. If more info is needed please ask and I will go outside and get the specs.