Well ..... here's my take on bioballs.
As mentioned above - bioballs are VERY efficient at converting ammonia to nitrite, and nitrite to nitrate, due to their design and placement in the system. Large surface areas typically allow for large nitrifying bacteria populations, and the abundance of oxygen present gives these aerobic microrganisms a great place to do their thing. They do indeed work - but again as stated many times by others, they provide little if ANY denitrification - thus IMO - using a wet/dry with bioballs as your only biofiltration "allows" nitrate to build up over time. Back to the old water changes to reduce these increasing nitrate levels.
Using a wet/dry with bioballs in conjunction with a DSB and LR does not seem to be the best route to go for following reason.
Once a saltwater reef or FO/FOWLR tank, upon completion of it's initial cycle, matures, and the number of fish/invert ammonia producers is held constant - will typically produce a "given" limited amount of ammonia waste. Obviously you want your live rock and deep sandbed to handle both the nitrification and denitrification of these nitrogen based waste compounds. By allowing your bioball wet/dry filter to still function in this scenerio, you in are in escence "robbing" the much needed ammonia / nitrite from the bacteria populations that live in and on your live rock, and the upper layers of your sandbed.
You only have so much of this waste in your system, and allowing the bioballs access to it, you may end up depriving the LR and DSB bacteria their primary food source. Where do you want the major concentration of bacteria? In and on your live rock and sandbed ... right. I don't think that will happen as well when you are also running your wet/dry/bioballs.
Lastly, I don't think it's necessary to point out that passing a given volume of tank water over a packed column of bioballs means that all ammonia and nitrite in that volume of water has been converted in one single pass, but I mention it anyway. I've seen wet/drys with several chambers of bioballs with other types of filtration media, be it foam, fiber, mud, skimmer... whatever ...sandwiched between the bioball chambers. I suppose one could say that as the water passes through the first chamber, some amount of the ammonia/nitrite is indeed converted by the bioballs. But in reality, in such a high flow area of the system, much of the nitrogen waste passes through. Contact time with the bacteria of the bioball chamber is not a "one shot" deal. It's continuous.
So ... if you have a wet/dry with bioballs on an existing mature system, with an established bioload of fish/inverts - and NO live rock or DSB - Keep the balls. You need'em.
If you are converting over to a FOWLR or reef tank - and have added a sufficient amount of live rock, and are in the process of establishing your DSB - you would be advised by many to slowly remove the bioballs over time as described above.
If you are at the initial stage of setting up a brand new tank - are going with LR & DSB as your primary biofiltration - AND - have a wet/dry trickle filter - most reefers would agree to not even use the bioballs from the onset. Just use the wet/dry as a sump for other uses.
This is by no means 'written in stone" though. Many reefers and hobbyists will use a combination of various biofiltration methods - and their reefs look beautiful.
It's very clear to me that there isn't just "one right way".
But if you want my opinion JohnnyFish, as long as you have enough live rock and an adequate deep live sandbed from the get go on your new 125 - lose the bioballs.
