Thanks for the quick reply.
I have a Tetratec EasyCrystal FilterBox 300. It has a coarse filter which it calls a "BioFoam" and says I should change rarely. As well as a finer looking filter, which basically looks like a fabric bag with something inside, which it calls the "EasyCrystal FilterPack cartridge". This is what it tells me to change every 4 weeks. Finally, there are some "Bio Filter Balls" in the bottom which it says can be used repeatedly.
So does that mean I can change the "EasyCrystal FilterPack cartridge" as there will be enough bacteria left on the rest of the components to cope?
The instructions for changing the "EasyCrystal FilterPack cartridge" are literally: take it out, bin it, put new one in. There's no mention of saving any of the bacteria at all.
I may be overly cautious but I don't completely agree with the previous posters, at least not quite yet...
First of all I would get over the idea that the brown sludge is horrible. It indeed includes your hard-won bacteria and it is really perceptive that you've had the forsight to be concerned about losing those bacteria! In the hardware section of TFF there are some experienced fishkeepers with very large biofiltration who claim to have gone extremely long periods of time without having to disturb the bio portions of their filters.
That said, yours is an internal filter on a smaller tank and it will be best practice to give it frequent cleanings (not necessarily media replacements) probably monthly. The main question becomes deciding which media in the filter is truly serving as the primary bio area and never doing things to that to cause mini-cycle disruptions in your filtration process. Smaller filters like this can be deceptive I think because, since the bacteria don't know any better and anchor themselves virtually everywhere in the filter, the entire media set can be serving as the biofilter. Certainly the bioballs and the sponges are playing a significant role, but carbon is pretty good at it too, even though the carbon isn't needed.
Carbon is a leftover from the 1960's, 1970's when the common wisdom was that "aged water" was a good thing. Although there are some good things about water that has been around a while, for the most part it was a mistaken notion confusing the water age with the time it takes to cycle a filter. Aged water often became yellow and was high in organics that caused odor, thus contributing to the feeling that there was a constant need for carbon in aquarium filters.
Reading here at TFF, you will find that modern aquarists consider carbon to be a special-purpose media that is kept on the shelf but not in the filter. It is used primarily to remove medications after they are finished working, to remove excess yellow tannins from new bogwood that hasn't been boiled enough or to remove unusual odors that are unexpected and short term. Carbon only works for about 3 days, after which it is useless. Although it will harbor good bacteria, it is a poor choice for biomedia because it crumbles to dust over time and the bacteria is lost.
Carbon is still actively promoted by companies that sell a variety of supplies in the retail setting because it is a steady revenue source once a new hobbiest thinks it needs to be regularly purchased. For fish shops, supplies that are bought regularly, like carbon (and "aquarium salt" is another one) are just too good an income source to let go of, so modern information is conveniently overlooked.
So, one thing you might plan to do is to gradually (gradually because it has good bacteria!) replace your carbon section with some form of modern small ceramic pebbles or other optimized biomedia. You would just have a bag of the new media and replace a little bit of the carbon at each filter cleaning.
Now, about that cleaning: sketchy is very correct about using -tank- water. Never kill your bacteria by using tap water on it! But until your filter is quite mature (yours isn't at 3 weeks since fish) you should be very gentle about this monthly "swish out", just gently swishing or very lightly squeezing it in the bucket of tank water. You want to get rid of the excess loose debris, but you want lots of gooky black brown bacteria to remain (maybe other members would have more helpful descriptions?) That basically describes it for a sponge, and I think for a modern sponge it should be a very long time before the sponge breaks down and needs to be replaced. For the bioballs it is more of a "dunking" in tap water is how I would describe it.
I know this sounds picky, and maybe other members will give me a hard time for the description but I think better safe than sorry at first and as the months roll by and the bacteria are more and more established, your sponge squeezings can get stronger without any harm to the robust size of the remaining population.
Anyway, that's my 2 cents, brought on by what an unusually good thing to have asked before acting!
~~waterdrop~~
the manufacturer recomended monthly replacements, but I ignored them 
