Agreed with Pest Control.
If your filter is 'mature' (over 6 months old) you can afford to move up to half of the filter media to the new filter and the mature filter should cope ok. A colony of mature filter bacteria can double in around 24 hours. Make sure to replace the media which you remove with new media.
That said, whilst taking half should be ok, I usually recommend one third to be on the safe side.
As PC says, there is virtually no bacteria in the water, but it is worth moving some water over as it will help to make sure that the water in the new tank has similar properties to the water in the mature tank, and will help to reduce the risk of the bacteria reacting negatively to the possible change in water parameters.
When you move the media over to the new tank, make sure to add some fish at the same time. You can roughly estimate how many fish you can add. For example, if you have 15 small fish in the mature tank and you move 1/3 of the media, the bacteria in the new tank will be sufficient to support approximately 5 small fish.
If at this point you only add 2 small fish, the remainder of the bacteria will die off and be wasted.
If you move over the mature media and don't add fish, all the bacteria will die off over the course of a few days.
Good luck. Let us know how you get on.
BTT
