how many bio balls?

[bre's dad]

I'm setting up a 120 fish only tank. I'm building a wet dry (have an extra tank, overflow & lots of plexiglass. Looking on the high side, figuring a heavy load, how much room should allow for bioballs? I don't have any formulas, but I'm guessing around 4-5 gallons (looking at other sumps).

 

[BillyZ]

have you considered yet a DSB and or LR rather than bio-balls? IMO, less maintenance and lower nitrates.