We started up our pool this year with little issue and it has been working great. We like everywhere around the midwest have had almost 100 degree weather everyday with little rain. This caused a ton of alge and cloudiness in my water. So my wife was using a ton of clairifier. I started to notice that the sand filter was at 40psi more and more. So I changed the sand filter sand and assumed it would fix the issue. Well during the time I was going to get sand and time to change the filter I noticed tons of bubbles or air in the returns. I shutoff the skimmers and the bubbles disappeared.
Anyways I changed the sand and rehooked everything up. It was fine for a few days but all of a sudden I have 40 psi again and no flow. I am still just coming off the main drain as I have not had a chance to look more into the skimmers. I am not sure what or where to check about this 40psi no flow issue? Any suggestion?

Thanks in advance,
Jason