Pool system pressure

[woofer]

My water pressure, measured at filter, has been low for some time- 9lbs normally -add 5lbs when rooftop solar system on- deduct 2 or 3 lbs when KreepyKrawley operating. I've done all the checks for blockages etc:
I am wondering if the pressure level may be affected because of low sand in the filter- it's an old Hayward
which is quite difficult to open up and look inside and even harder to change the sand.
I 'think' it's functioning reasonably well although I've had a couple of algae incidents recently which could be due to reduced circulation!
I have never seen any mention of low sand level creating pressure problems but I have just checked out the pressure when on 'recirculate' ( assume the water is missing filter and sand) which was about the same as running on 'filter' normally.
I would appreciate any thoughts on the situation before I panic and open up (with difficulty) the filter to see inside.
Thanks, Woofer