Hmm okay so what i read so far is that...copied from another site american aquarium products concerning both...
Carbon is often not necessary in established aquariums especially tanks with regular water changes and plants. I generally only use carbon to remove chemicals after treatment then remove the carbon. You can also leave old carbon to become a nitrifying bacterial colony. This point about carbon also lends credibility to Sponge Filters which are often considered poor filters due to the fact they provide no chemical filtration, this is based on poor information as to the need for carbon filtration.
[6] Simple to clean, simply remove the sponge and squeeze or rinse in old aquarium water or de-chlorinated tap water. The advantage here is the aquarist is less likely to ignore cleaning the filter (unlike many complex filters), which can lead to organic build up, increasing nitrates and lowering KH and ph.
[8] Probably the best choice of filtration for breeding tanks, Betta tanks or bowls, hospital tanks, and even discus aquariums, due to the fact that they do not “suck” in baby or weak fish or attract noxious bacteria.
[9] Easily the best filter for a small tank or bowl, especially for bettas with there long fins that can get caught on intakes of HOB or other filters. They excel over UGF in bowls or small tanks in that they are easier maintenance, and more efficient biologically and mechanically.
so what i gather is that...its fine to leave the same one in,as long as you clean the sponge every now and again. Now this filter i have cant exactly take apart,but i could swish the whole thing in water...and you can leave the carbon in as well,as it can hold nitrifying bacteria as well...
also read with a small bioload ( as in bettas) it doesnt have to replaced nearly as often. Says 4-8 weeks,so two months at the most. The sponge will start looking real funky,the water might not clear up as much as it did before,might start smelling a bit,then thats when you change it. But it should last a good while with just one betta fish.
HTH and didnt confuse you anymore lol!