A filter, along with a heater, is probably the most important part of any fish tank set up. Without it the water in your fish tank will become much like a stagnant pond and little will be able to live in it except for a nice film of green algae scum.
There are two main types of fish tank filters; external and internal. Both of these types of filters can be used in either fresh water or salt water aquariums.
When it comes to internal filters there are two basic types. The first is the under gravel filter seen primarily in fresh water tanks. This consists of a raised plastic grate that sits on the bottom of the tank. The gravel or base material is placed over the top and clear vertical tubes attach to holes in the filter. These tubes have either a power head sitting on top or a bubbler inserted into them. The water is forced to flow through the gravel and back up the tube into the tank. The gravel itself is the filter, collecting the extra food and fish waste that is pulled down into it.
The second internal type filter is more along the lines of a sump tank just below the main tank and is used mainly for salt water tanks. There is an internal tube that that allows water to flow down into the sump where it is pumped through a series of filters that catch all the waste and bad bacteria. The filtered water is then pumped back to the top of the main tank keeping a continuous flow going. This is a good filter choice but it is on the pricey side and does create quite a bit of background noise.
An external filter is the best choice for any fish tank, if you can afford to use it. All the filtering is done outside the tank, keeping the filtered water separate from the unfiltered water until it is returned to the tank. A couple of long hoses connect the filter to the tank, one that supplies the filter and the other that returns the filtered water to the tank. This type of filter is the most effective but also the most expensive.
You can purchase any of these types of filters at your local fish shop, but may have a harder time finding the more expensive external type at a general pet store. The sump type filter will require a tank that is either altered or custom made. If you are going to spend the extra money on this type of set up it makes better sense to spend the money on a good quality external filter instead. An external fish tank filter will cost more up front but in the long run you will save on maintenance costs and fish costs because you will be providing the best habitat they need to live long and healthy lives.
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