OK, so Willy, about one of your earlier questions: Having filter media in little bags can be useful in various ways. In an HOB (aka "waterfall") filter, it can be useful because you can lift a whole section of media out of the filter and dip in in tank water and clean it as a bunch. (The down side to that is that some mesh bags trap more of the debris than you might like and actually make it a little harder to lightly rinse the debris off.)
Another function that a mesh bag can play is to keep two dissimilar types of media apart from each other. This is particularly useful for crushed coral (broken up bits of shell and coral that are used to slowly raise the KH and pH of a tank) which one might want to later remove and which might need to be rinsed more often to keep it effective. This particular use I consider the most legitimate use for bags.
Yet a third very minor function might be to be able to know the age of a particular bunch of media. Some people might want to "age-out" (although usually no real reason to!) a particular media but they periodically buy the exact same type of media. This way they would know which batch (pebbles, rings, bioballs, whatever) had the same age. Note that this is not something I am saying really needs to be done.
BUT, I believe that for the most part, as you get more experienced as an aquarist, you hesitate less and less to just cut those little mesh bags open and do whatever the heck you want with the media inside. Most people just have areas of loose biomedia (referring here to the rings, pebbles and bioballs rather than sponge) and pour them into a platic tub or cup when cleaning them in tank water I would say or have some other technique.
Anyway, a bit long, but hopefully some bits to think about since you were on the topic,
~~waterdrop~~
