Ditching the bioballs is excellent advice, however you cannot do that until you replace it with live rock. Assuming that your tank has already been running for a couple months (completed the intial cycle) you should start adding live rock into to the display. If the rock is already cured (been in a running system for at least a month, preferably longer) you can add as much live rock as you can afford.
If you are adding un-cured from your lfs I would suggest that you add somewhere around 10-15lbs at a time at least a couple weeks apart if you already have tank inhabitants. The die-off that will occur from newly imported rock will de-compose and cause a mini cycle increasing nitrates. If too much is added at one time you will be at risk for having measurable amounts of ammonia as well as nitrite which will be deadly to your tank inhabitants.
Ideally you should test for the aforementioned three weekly (or more frequently) while going through this process, only adding more rock when ammonia and nitrite tests at 0. Ideally, during this process you dont ever want to have measurable ammonia or nitrite, though you likely will, especially in a newer tank with just bioballs. You have much more leeway with nitrate, however high levels will make you algae problems worse. Personally I wouldnt want nitrates to rise above 20 during this process. That number is subjective because with bio-balls you likely already have high nitrates.
If your tank is un-inhabited then dump as much live rock in that you can afford and let it run its cycle and cure at the same time.
With either method you could start slowly removing bioballs each week, after you finished adding your live rock, safely say 20% each time.
A filter sock is not a replacement for bioballs an any way whatsoever. The bioballs are there for colonizing bacteria to convert ammonia to nitrite then to nitrate. The problem with bioballs is that the pin design (to maximize surface area) traps detritus, the end result being more trapped detritus and even higher nitrates. In addition to simply not having enough area to house as much bacteria as you need in a reef, bioballs will not export nitrate. The anearobic core within live rock will colonize with bacteria that converts nitrate to nitrogen gas. A deep sand bed can also help with this, however thats a wholenother discussion and comes with its own drawbacks.
