Hi can anyone advise me please on an issue I've had now for 3 weeks and its driving me insane.  I've got a 12ft round above ground pool (9150 ltrs) set up with an intex sand filter which I've had from new for 5 years now and I changed the sand 3 years ago in it , In total I've had a pool set up for 12 years so I like to think I kind of know what I'm doing. However I'm stumped now i've never ever had any problems with recurring green algae on the bottom but for some reason it's happening now. The last 2 weeks I've managed to stabilise the TA as it was high which got the ph perect, cholrene levels perfect, but time and time again the green reappears. I've been regularly putting in anti algae weekly and added claifier to clump the green together and hosed it out with my filter hose brush. Ive also shock cholrened it many times since I set it up for this summer, I've done backwashers on the filter daily to remove the algae from the sand and rinsed it afterwards for 1 minute. I've been checking daily now for a week, hosing the bottom and running the pump daily for 6 hours sometimes more and I know it should only need running 4 hours a day so I'm not skimping on using the pump. The pool was used yesterday by the family it was perfect all the readings were within the range as I had hosed out any settlement on the bottom every day this week sometimes twice a day. Friday morning there was no algea at all I thought I have finally cracked it. however I went out to check today (Saturday) and once again the algae was sitting on the bottom urgh. Now this is where it gets confusing i read online today if the sediment window is dirty/green which mine is the o ring needs changing as it could be split or perished. Could this be affecting the problems with algae I'm having maybe ? It's not leaking from the 6 way valve so I dont think it is but I would like that confirmed please before I take it all apart to look. sorry for the long post but any advice would be appreciated before I actually lose the will to live and pack it all away again which I really dont want to do. Thanks so much for taking the time to read my long post