id try letting the new one soak in the tank for a week or so hide it somewhere so it gets a chance to get some bacterria on it either that or look for a new filter cartriges i have a wisper filter in my 10 gallon the filter that it comes with is an all in one dealie the lady at the petstore had one that isnt all in one theres a sponge and the carbons seperate i got one of those instead but if thats all you got let it sit in the tank for a while first before switching it over