Since I started the tank so 3 weeks. It says the wheel is supposed to colonize good bacteria (I don't know what that's called exactly, I don't have any of it with me) but I dont see anything on it. It looks like the water just falls off of it and doesn't absorb it (like a bids feathers). I wish I could word all this better lol- basically it seems like the filter cartridge would do a better job but I don't know much
I would only replace the cartridges IF they need replaced due to falling apart or because they are so crazy dirty that they can't be cleaned. The bio-wheels are not that good and often fail. If it does and you replaced your filter media then you just lost most of your bacteria. So in other words, there is really no good read to change them except that they are falling apart or in some other way failed. All you need to do is clean them in tank water when you do water changes and replace. Nice and easy and saves the cost of buying new media all the time.