CF40 - Ciano
ciano.pt
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Do you not have the green edged Bio-Bact cartridge? If you do, you have two places for the bcateria, the Bio-Bact cartridge and the foam, which most of us call sponge.
The instructions say to change the Water Clear (carbon) cartridge every 30 days; the foam (sponge) every 90 days and the Bio-Bact cartridge every 140 days. This is typical of instructions as they want your money. Sponges (foam) should only be replaced when it starts to fall apart after several years.
They don't say what's inside the Bio-Bact cartridge but the name implies it is designed to be a home to the filter bacteria.
Sponges (foams) should be washed by squeezing them in old tank water taken out during a water change. The Bio-Bact cartridge can be cleaned in old tank water by squeezing if it is soft or by swooshing it round if it is rigid.
You can just leave the blue edged carbon cartridge there if you want. Carbon is used to remove things from the water, usually medication after treatment has finished, or the brown colour that often leaches out of wood. Other organic chemicals will be removed as well. These things all stick to the surface of the carbon (they adsorb rather than absorb), and eventually there is no room left for anything else. This why filter manufacturers tell you to change carbon regularly.
Decades ago, carbon was used a lot. Water changes were not done regularly as they are now and the water turned yellow. carbon removed the yellow. But it is cheap to make and if you have to replace it, it's a nice money earner for the manufacturer.
We now know that carbon isn't necessary but filter manufacturers still insist that it is and most of them include it as one of the media.
If you just leave the cartridge there, it will quickly become full and stop adsorbing things and be just a home for the bacteria. However, if the filter also has a sponge and a green topped cartridge as the instructions say, you can take it out now and replace it with another foam (sponge).