After 4 weeks, my 55g and 29g tanks are done cycling. I did not use live rock/sand, but I added Microbe-lift Special Blend bacteria as instructed. I have a mix of Marineland Penguin, Aqueon, and Tetra hang on the back filters. When should I change the filter cartridges (I plan to have 1 new cartridge running on one filter in the tank, and an old cartridge in another filter)? When can I start doing water changes?