I hooked up a new sand filter and pump today. After dealing with a cartridge filter that was to small for to long. I vacuumed the pool and the pressure went from 12 psi to about 18-20 psi. I have read that you back wash after I think its 20% rise. Could 1 vacuum job do this? ( The pool was dirty) And should I expect after I vacuum to back wash. (is this normal?) The pump and filter are a Hayward 1hp and 18 inch sand filter. The reason I noticed a problem. My fountain was not going very high in the air. After I back washed it returned to normal/