I went to the pool store today to get a skimmer sock. They guy there said the skimmer sock won't catch fine particles such as algae. Instead he sold me some cullulose fiber DE replacement that you put into your skimmer to help your filter grab the fine particles. He said to add it 1 scoop at a time to my skimmer until the pressure goes up 1psi. I told him that my pressure never seems to increase and he said I may need more or new sand. So I went home and after adding 5 scoops my pressure didn't budge. I called the guy and he said add 2 more scoops and if it still doesn't move then I probably have a problem with not enough sand or needing new sand.

Can the kind folks here please give their opinion on the pool guys' diagnosis? If I do need to add sand or new sand is there a guide on this site somewhere of how to do that?

Thanks in advance