The bed could be getting close to needing a change out, one good sign is the with the water at the same quality and usage the same the system has to go more often for the water to be treated.
Yours would be about 10lbs of fine pea gravel or some thing along the line of #20 and then about 1 cubic foot of carbon, at least I think that is what culligan uses in systems like yours.
The amount would depend on the size of the media tank under the control that you have worked on.