By the way, NEVER change the filter cartridge. It RUINS the cycle of the tank. A filter cartridge takes 1 month to begin working, and then it is called "cycled" The bacteria that grow in in eat ammonia and produce less toxic, nitrate, it is only toxic in high levels. There are proceses in between, btu I won't get into detail. Anyway, the bacteria live in the filter and everytime you take out the cartridge, you are throwing away all that beneficial bacteria!