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There are a number of reasons for this, firstly I would check the multi valve on the sand filter is set to filter and no bypass.
I would then try and add a enhancer\clarifier to the water leave it running for 24 hours then do a long back wash and see if the water clarity improves, this can be purchased from any pool store or online such as the one here.
If this doesnt improve the water to the quality you require I would look to shock the pool to around 10ppm, leave that for 12 hours and then bring it back to a safe bathing level.
After shocking the pool if the water clarity is still not of a high standard, I would consider replacing the sand in the PT900 filter, this should be done every couple of years, and if it hasnt been done recently its worth doing.