I've done reef tanks with both and have found bio balls to be completely unnecessary. with good quality live rock and adequate circulation there is no need for them. You may see them in some types of refugiums but I think that is mainly to keep the mavroalgae from over growing and getting stuck in the pump.
I've personally used bioballs in all types of setups; FO, FOWLR, and Reef. All of which is more of a hinderance than a help later in a tank's life. At first during the initial cycle and a few months afterwards, it does help. But any point beyond that and it will start releasing excess nitrates. There are successful tanks with BioBalls and no Nitrates, but these tanks usually have many large methods of nitrate reduction including macro algae, deep sand beds, lots of live rock, and many water changes. But, that's just because the nitrates are undetectable. The BioFilters (BioBalls, BioWheels, etc) still release nitrates, but the other forms of nitrate removal take it up as fast as it's released. It's really a delicate balance. Anything that would add too much nutrients would send the tank into a nitrate rich environment. Heavy bioload, adding large messy fish, letting a dead animal decompose, or even accidently spilling food could cause it to become imbalanced. Usually, if you are using a BioFilter filtration method, and your tank cannot cope with all the extra nutrients, you will have a tank full of algae.
<a href=showthread.php?s=&postid=8095888#post8095888 target=_blank>Originally posted</a> by Sk8r I used them back when they first came in fashion. My new reef works way better without any filtration at all.
i have used them in the past also and really i couldnt tell what they did if anything. I have been running a filterless reef for two years now and everything is going great but you know how us reefers are ....always wanting more junk that we dont need, i built a custom all glass 60inchx18x15deep sump/refuge with protein skimmers, carbon, macro, deepsand bed and more. Hope it doesnt mess everything all up now!!!!