04-04-2011, 03:11 PM
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unless its falling apart like mush, rinse it off with tank water.
If you HAVE to change out the cartridge, you can put in the new cartridge and squeeze the old cartridge into the filter so a bunch of that mulm and bacteria get stuck onto the new foam. A lot of your bacteria is in your substrate and plants and such, at worse, you will get a mini cycle. Unless you have ultra sensitive fish, its unlikely it will hurt your tank that much.
Personally, I would go for one of those filters with the foam + bag of media + bag of media setups. Allows you to staggers media changes as to avoid a new cycle. A friend of mine fills his HOB, a old tetra whisper filter that also only has a cartridge, with ceramic filter media. Basically does the same thing as the cartridge unless you need the carbon and he changes out half the media every half year when they get too yucky.