We just started second season of using our 15', 5,000 gallon above-ground pool with Hayward Xstream cartridge pump. After we opened the pool and ran the pump, I took a water sample to the pool store and they gave me a list f chemicals to add, including 1 pound of shock, which I did. The next day chlorine was 0, other chemicals within normal range. I added another pound of shock, in the evening the chlorine was again 0. Last night I took another water sample to the pool store and they told me to add 3 lbs of Alkalinity increaser, about 1/2 lb of the stabilizer thru skimmer and 1 lb of shock, which i did and left the pump running all night. It rained a bit overnight. In the am chlorine was again 0, Alk was up and pH was down, a bit below normal, as intended. CYA is about 0. I then added another pound of shock. After checking the water 30 min after adding the shock, the result is very low chlorine level (about 1). Should I keep adding shock or should we check the filter next? It was clean after my husband put it in before starting the pump. Is there a wrong way to install the filter that it may be malfunctioning? The water looks pretty good, just a bit cloudy, but we can see the bottom of the pool. Any help will b greatly appreciated. Thank you!