I'm going to vote for keeping the bioballs with the caveat that you also need sufficient LR and DSB.
Pros
1) Mechanical filtration
2) Biological filtration. Nitrate isn't as bad as nitrite, ammonia. Bio balls do this very well.
3) Silencer - cuts down on water noise.
4) With sufficient LR and DSB, your denitrifying bacteria will take care of your Nitrate.
Cons
1) Must rinse them occassionally.
Note that LR with insufficient waterflow will also produce nitrates in deadzones.
It's always about finding the balance.
I agree that rubble is better as it provides the porous areas bio balls can't.
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