It’s normal for the resin tank to be around 2/3 or 3/4 full of resin. This allows for the resin to move around during the backwash cycle.
If you need to rebed your resin, it’s very important to leave that space because if there’s too much resin in that area it can lead to lower water pressure and potential damage. Refer to our article Replacing Resin Beads for instructions if needed.