New Filter

Hi deano,

If you get a filter that takes the same type of media ( although i'm not sure this is essential)  you can put a bit of your old filter media in your new filter and this will seed it. You still need to allow the tank to cycyle though before you add fish.

FC76

You can run both together but you need to leave them both running for at least 6 weeks, and even then when you remove the old filter you might well see an ammonia and/or nitrite spike.
As Fishcake says, it is much better to put all the old media into the new filter any way it will fit.

Is this the tank with the platy with whitespot? If it is, it might be worth waiting until the whitespot is gone before doing anything with the filter