Originally Posted by Animal Liberation Front
so whats the difference in a normal hang on filter and a canister filter and a sump or overflow or wet dry... whatever? why does a hang on work for fw but not sw? what does if have to do that it cant?
What you list are not actually filters. They are methods of moving water. How you choose to utilize each of these devices determines the type of filtration. There are 3 basic types of filtration, which are mechanical, biological, and chemical.
Mechanical filtration, also called Particulate filtration, occurs when water flows through a filter pad. The filter pad traps large particles, helping to make the water appear clearer. This is a visual benefit, but does very little to help the water chemistry. More advanced studies of saltwater systems will show that mechanical filtration actually lowers water quality, unless the filter pad is cleaned daily. I personal do not use mechanical filtration. Most hang-on filters and canister filters utilize mechanical filtration as their primary function. Wet dry (or sumps) systems usually have a filter pad in place also.
Bilogical filtration is what keeps your fish alive. Without biological filtration, ammonia accumulates to toxic levels. Biological filtration is achieved by allowing bacteria to grow on bioballs (sump systems), sponge pads (hang-ons or canisters), biowheels (hang-ons), or in the gravel bed (undergravel filters). In freshwater systems, biological filtration breaks down ammonia into nitrite, and then nitrite into nitrate. Nitrate is less toxic, and is removed by frequent water changes. In properly set up saltwater systems, biological filtration exists in the live rock and live sand. The sand and live rock biologically convert nitrate into nitrogen gas, which is non-toxic and leaves the system naturally, without water changes. The use of biological filtration in saltwater systems should be limited to only live rock and live sand.
Chemical filtration assists this entire process. In freshwater systems, activated carbon removes organic acids from the aquarium. These acids cause water discoloration and disrupt the pH stability of the system. Hang ons and canisters both use activated carbon. When using a sump systems, activated carbon is often used in the sump area. In saltwater systems activated carbon is also useful for these reasons. Additionally, by helping keep the water clear light is allowed to penetrate to greater depths for coral benefit.
Finally, the best form of filtration ever invented, which you do not mention. This would be the protein skimmer, which is only used on saltwater systems and is technically a chemical filter. The skimmer is a plastic cylinder which hangs on the aquarium or is placed in the sump. The skimmer produces tiny micro bubbles. Organic waste bonds to these bubbles and is REMOVED directly from the aquarium. Because the skimmer is so efficient, supplemental biological or mechanical filtration can actually lower the water quality, because the skimmer is not given the opportunity to remove the organics. I could write an entire book on the benefits the skimmer provides, but luckily there are already hundreds of such books on the market for you to read. If you have a saltwater system and you are not using a skimmer, then you are ignoring the most critical piece of advice given about marine setups.