I have never used bio balls but have used the ceramic rings in fluval filters before and fought with nitrates all the time. On my current systems there won't be a bio ball to be seen.
Your tank has only been set up for 8 weeks so don't know that you will have much of an issue taking them all out at once but just to be safe I would take out 1/3 a week for 3 weeks and replace with LR rubble.
Depending on what type of live rock you have - want to go with - will determine how much more to add.
As an example I have about 250 lbs of Fiji and 250 lbs. of tonga in my 230g with a 77g. sump/fuge. There might be a bit more in there as I have added some misc. over time.
Don't know if that helps - just what works for me.
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