This may only answer part of your question, but I don't believe I will be leading you down the wrong path. Has your chlorine been a constant 2-3 ppm? That is low for really shocking your pool. I will let some of the others weigh in here, but I believe I remember reading (and successfully following the directions last year) on the board before all the history was lost, that in order to really kill algae, you might need to be around 20 ppm if you have an outbreak, until you get a handle on it. How warm is your water? Have you isolated it to a filtering issue, or could it be an outbreak that you aren't getting a handle on because your chlorine is too low? Hopefully some others will weigh in, but I think you might be focusing on the filter and it could be a chemical/chlorine issue. Thanks.