10-21-2011, 07:30 PM
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It has to be the media that was used in the old filter that the water ran through, if its a carbon pad then use this, if theres also a sponge then use that too, if it doesn't fit then cut it down, if cutting it down leads to carbon coming out then throw it in a media bag so the bacteria can still do their job and filter the water. Just fyi in a healthy tank carbon is not really needed so if you want skip the carbon from now on and just use filter floss or other bio material that will work fine and will be cheaper.
Key is to remember that beneficial bacterial colonize "hard" surfaces, not the water so you will lose some of the bacteria by replacing the filter out as some of them were colonizing the walls of the filter, but compared to your bio media material this should be insignificant if you just make sure to get all the actually filter material into the new filter, until your new filter material has time to be colonized by the bacteria.