Hello everyone, I have a newbie question when it comes to filters. I have a top fin power 20 filter for my 20 gallon tank and was wondering if I am suppose to replace the filter cartridge each month? The filter cartridge also came with carbon already in it. I know that I am not suppose to remove the bio-filter since that is what houses the beneficial bacteria. I just wanted to ask on here before i made a newbie mistake just in case i am not suppose to replace the filter cartridges. Here are a couple pictures of my filter cartridge and bio-filter.