The Aqua one 4c carbon and polymer wool cartridge is one of our best selling products for a good reason – it works exactly as it should, trapping fish waste and purifying aquarium water for 4 to 6 weeks after which it gets thrown away and replaced. What could be simpler?Aquarium Maintenance Made Easy…The 4c cartridge itself is however quite a clever little thing designed with complex aims and objectives in mind. The polymer wool top layer is carefully manufactured to be a specific thickness – dense enough to trap a month’s worth of solids yet not impede water flow. The polymer layer is also carefully bonded to the carbon tray, in such a way as provide support and prevent sagging when it gets dirty.The carbon tray does indeed hold the carbon granules which are an essential and very purifying but can also be messy if left to roll around. Water always attempts to take the easiest path through a filter so the carbon tray not only neatly contains the carbon but also ensures that water is evenly distributed throughout the cartridge making the most efficient use of the available filter area. Water then leaves the cartridge in an even manner meaning that the black sponge layer and bio-noodles are also utilised in an even, efficient manner.Beneficial BacteriaThe Aqua One 4c carbon and wool cartridges are not designed to grow the beneficial, nitrifying bacteria so often sought after by the fish keeper. Rather the 4c is around to stop harmful solids and excess chemical waste reaching the foam and Bio-Noodle layers which will grow a nice thick layer of good bacteria.4c cartridges don’t always last the full six weeks in between changes and it’s worth checking on their performance once per week. Just lift the filter lid and peer in. If water is ‘pooling’ on the surface polymer wool layer rather than passing straight through, the cartridge is blocked and should be changed. Don’t attempt to wash the cartridge under the tap, it has been used up – change it.Change every six weeksWhile the 4c carbon and wool cartridge needs changing every six weeks, the foam and Bio-Noodle layers are much longer lasting. However there is a limit to how long even these well designed items last. So, change a few Bio Noodles every month to stop them all becoming clogged up at once and change the foam/sponge layer yearly at least. In between, the sponge layer may be rinsed in a little aquarium water, which is then thrown away of course.