Nitrifying bacteria builds up fast in filter socks, because of the micron count, there's literally more surface area on a filter sock than all the rock you could imagine piled into a tank, similar to marine pure bio bricks. It could have been housing a small portion of your systems biological filtration and when it was removed the system went through like a mini cycle and stabilized after a little while. I say it builds up fast based on the fact that you need to change/clean them often. And I honestly kind of lose the distillation between why you wouldn't use a piece of filter padding or sock as biofiltration media instead of spending the big bucks on ceramic bio bricks like marine pure, although I think it might have to do with the ceramic being inert. I read a while ago medications can't bind to ceramic media, going off that I always assumed it was good to have a few bio-balls with the sponge inside incase I ever needed to medicate the tank, although if you have too many bio-ball sponges they also become nutrient factories themselves I've been told. So again I kind of lose the distillation between why ceramic media is so effective at biofiltration, without it being considered a potential nutrient factory given it's surface area. Sorry for going on a tangent lol.
