The valve that they used is a Fleck 6700 older style or first gen digital control.
I would put the link for the control but fleck has removed the old and replaced it with the new one so drop a pm and I can send the
manual for your 6700 ,
Next, get a water test for the hardness, Iron and ph.
The size of the media tank looks to be a 10x47 but use a tape, across and floor to top of media tank or base of the 6700.
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