If the cartridge becomes too old or starts to fall apart, put a new cartridge in the filter but keep the old one behind it for a week and monitor for ammonia spikes.
Non cycled tank doesn't meant it not livable:) I don't cycle my 2.5 and 5 gall tanks.
But anyway, i don't have experience with cycling but i have someone explained it to me, so i am going to post her explanation for you. Don't remember if she wrote about filter media but you always have to do weekly water changes, vacuum the gravel and rinse/swish the filter. Not that what she wrote to me:
''So for the Nitrogen Cycle, what happens is you put fish in the water, the fish poops and excretes other dissolved organic compounds which is ammonia. Without a cycle that ammonia just sits there and builds and builds until the fish keeper does the water change, so the longer the tank is left there, the worse it can get.
So in order to make it safe for the fish and to not have to do 10 million water changes a week, you do something called a cycle. During the cycle you are growing a colony of Beneficial Bacteria or BB (**Note** you can't do this without some kind of filter!), now that BB needs something to eat in order to grow! So you give them ammonia and they eat it.
They also then turn the ammonia into nitrites which are also bad for fish but soon they turn those nitrites into nitrates. Nitrates are not as deadly but if they remain above 80-100ppm they can be just as deadly as ammonia/nitrites. So You have your BB, they start turning ammonia into nitrite and nitrite into nitrates. Nitrates are eaten up by plants and also taken out by water changes, that's one of the reasons that water changes are still very important!
So in order to get the cycle going you need a source of ammonia, for most it's your fish! So you have your fish, your fish poops and you monitor the ammonia everyday for a month. When you test and your test reads higher than .25ppm you do a water change, I usually start with 25% at first, wait and hour after you change and then test again. If it's still over .25ppm's then you do another change to get it at or below .25ppm. Normally you don't have to do a water change everyday but you should still monitor each day just to keep your fish safe during the process.
So finally after watching ammonia, nitrites will come into play. You just keep watching your ammonia and do a change when it gets over .25ppm, then nitrates will come joining the party!
So how do you know if your tank is cycled? Say one day you test for all three, for ammonia you get .25ppm, nitrite is .5ppm and nitrates are at 40ppm. The next day you test and you get 0 ammonia and 0 nitrite, that means your cycle is complete!! It has to be within a 24 hour period though, and nitrates will still be high, probably higher than 40ppm's so that's when you do a large water change at 75% or so and that should get the nitrates down to a manageable level. Anything at 20 or lower ppm's of nitrate is good and safe.
So after all that, your tank is cycled and now you only need to do a water change once a week, usually around 50% will suffice. You've successfully grown your colony of BB and they're working hard to keep your ammonia and nitrates safe! So don't let your filter dry out or they will die. BB grow on every surface in your tank, they do not reside in the water column, well one strain does but it doesn't do anything for your cycle. Those BB are actually the ones responsible for a bacteria bloom in your tank if you see a white cloud. Those can just be taken out with a water change and don't hurt anything, just a bit unsightly.
And don't worry about rubbing them off or anything, they're stuck to things like super sticky glue and only come off if you let it dry out. They are micro-organism's so it's not going to clog your filter or anything. Each week when I do a water change I take the filter cartridge out and give it a swish and rub in the old tank water I just took out to get rid of debris and non-dissolved organic compounds such as detritus/mulm. I only replace filter cartridges when they seem to be falling apart, no sense is buying them each month when there is nothing wrong with them. When you do get a new on, if you can stuff it in the filter so that it can get to work housing new Bacteria so that when you do take out the old one, you don't take out a load of bacteria when you do! If it happens that your cartridge dies completely and you have to replace it on a whim, you might experience a mini-cycle if there aren't enough bacteria in the rest of your filter/walls of the tank/gravel/and other ornaments. Basically you'll just get a small spike of ammonia and in a few days your colony of bacteria will be strong enough to convert all of your levels back to safety.
Also, if you want to skip this entire thing, you can go out and buy a ton of plants, I usually keep it to the same amount of plants per gallon. So if it's say a 10 gallon, I like to get about 10 plants, more will certainly be better. But basically then you'd be doing a Silent Cycle, basically the plants eat up everything and you'll never get readings of ammonia or nitrites, they are a buffer for your fish. So this is why I like planted tanks! I never have to actually worry or work for my cycle to happen! Also getting plants from already established tanks help even more because they have beneficial bacteria on them already so it's giving your tank a "kickstart", of course you have to buy the plants if you don't already have them and they can be expensive. But in the long run, at least for me, it's worth it. besides they give you babies and grow unlike silk or plastic plants . Always remember to quarantine plants before you put them in the tank.