I have a Tetra Whisper Advanced PF 30-60 for my 30 gallon tank. It is currently cycling with no fish. There are 2 charcoal filters & 2 filter cartridges. I'm pretty sure that I should only change one side at a time to keep the good bacteria in one side while the new one acclimatizes. But how often should I change them, when I do need to change them? My hubby opened the box and of course tossed the instructions > I know I don't need to change it now, but I would like to buy a spare set to keep on hand just in case.