My girlfriend just got a fish tank. It's a 5 gallon all set up ready to go. Minutes the fish. We are going to do a fish less cycle and I figured getting a new filter for my tank wouldn't be a bad idea because then I could use my old established one in her tank to get the cycle moving. My question is how long should I leave my current 3 month old filter and my new one together before I give her my older one. I've heard that when switching to a new filter cartridge you should seed the old one with the new one to get the good bacteria transferred. Is this true? Am I on the right track? Anything helps, appreciate it.