i started out with second hand tank and second hand filter. its a little sponge filter. i am considering upgrading to something like a top fin power filter (i have a 10 gallon tank), since the little sponge doesn't do much to help keep the water clean and such. my main question is, can i do this to an alreay set up tank? i've had them set up for about a month now, water is jsut right, fish are doing great. will a new filter, that hasn't been cycled and what not, be a bad thing? what do i need to do to make the transition a smooth one? 
