Before you purchase a filtrations system for your aquarium there are a few basic items that you should know to help you make the best purchase for your tank.

1. There are three main components to filtering your aquarium water.  The first component is biological filtrations or the breaking down of the fish’s waste product by beneficial bacteria.  The second is mechanical filtration or the trapping of larger particles (fish waste or excess fish food for example) in a filter media such as a filter sock or sponge.  Lastly is chemical filtration which is the removal of odors, discolorations and dissolved organics by absorption or adsorption by media such as carbon or zeolite.
2. There are many different types of filters available to use on your aquarium.  The four most popular filters available include the hang on the back filter (or HOB), canister filters, wet/dry filters (or trickle filters) and under gravel filters (or UGF).
1. HOB filters, as the name implies hang on the back rim of your aquarium.  They will pull water into the filter, have it flow through filter cartridges and then spill back into the aquarium.  HOB filters tend to be one of the most popular filters as they are very reasonably priced and work very efficiently.
2. Canister filters will sit beneath your aquarium and use an inlet and outlet tube to bring water to the filter and back to the aquarium.  Although canister filters tend to cost a little more than HOB filters, they also tend to be much more efficient at filtering the water as they are able to hold more filter media.  Also if noise is a factor most canister filters will run almost silent.
3. Wet/Dry filters are generally used on larger systems (50 gallons and larger) as the filter itself tends to be larger in size.  Water will flow from the main aquarium via an overflow box in to the wet/dry filter where the water is trickled over a biological media.  A return pump will then pump the water back up to the main aquarium.
4. UGF’s are simply a slotted plate or series of plates that are placed underneath the gravel in your aquarium.  There will be a lift tube sticking out of one of the plates that allows water to flow up through the tube (using a water pump or air pump) in to the aquarium then through the gravel and back up the tube.  The gravel will act as the filter bed to trap detritus and for beneficial bacteria to form on.  UGF’s were very popular many years ago, but with the advancement of the technology in HOB, canister and wet/dry filters most people no longer use them.
3. How do you know what filter system is best for your tank?  While cost will certainly be one of the main factors, you should also look to see if the filter is addressing all three components of a good filtration system.  Many HOB filters will provide good mechanical and chemical filtrations and average biological filtration.  Canister filters generally will provide good mechanical, chemical and biological filtration.  Wet/Dry filters will provide good biological and average mechanical but many will not come with any chemical filtration so you will need to add that separately.  UGF will give good biological filtration and average mechanical filtration and no chemical filtration.  Generally speaking HOB or canister filters will give you the best bang for the buck.
4. So what size filter will you need for you tank with so many options?  Most manufacturers will give you an estimate on the size tank their filters will handle.  These recommendations are fairly accurate, but many times are figured under ideal conditions (i.e. no overfeeding or overstocking and you are doing your regular maintenance).  It is suggested to go with the minimum tank size recommendation and not the maximum size.  For example if you have a tank that is 30 gallons and you are trying to decide between two filters.  One is rated for tanks between 30-50 gallons and the other is rated for tanks 20-30 gallons.  The one rated for 30-50 gallons would be a better option.  Also ideally you would like the filter to turn over the tanks water volume 5-10 times per hour.  So for that same 30 gallon tank, the filter should flow 150-300 gallons per hour.
5. Your filter will need to be cleaned about once a month so when choosing a filter make sure it is one you are able to maintain easily.  While wet/dry filters take relatively no maintenance UGF require frequent cleaning of the gravel (or vacuuming with a gravel siphon/cleaner) to prevent build up of detritus in the aquarium.  HOB filters are fairly easy to maintain and only need their cartridges replaced and an occasional cleaning of their intake tube and impeller. Canister filters will require to be taken apart to get to the media inside, so they can be a little more difficult to clean.  But once you get the hang of maintaining any of the filter systems you will find it becomes much easier over time.
6. During the monthly cleaning process of your filter you will also need to replace some of the media.  Generally the biological media will last at least 6-12 months before it needs to be replaced (Tip:  never replace it all at once; you will want to replace about ¼ of the media at a time).  The mechanical filter which are normally sponges or floss can be rinsed monthly and then changed about once a month to every 6 months.  Sponges generally will last longer than floss.  Chemical media such as carbon will last about a month’s time, but you will want to check the manufacturers’ recommendations and replace as needed.  The cost of replacement media should be factored into your decision of which filter to choose.
7. If your tank is large enough where a single filter is not large enough or if you want to put more than one filter on your tank for better flow this certainly is possible.  Many times in longer tanks a single filter will not provide water movement through out the tank and you may find detritus builds up on the opposite side of where the filter is due to lack of flow.  In these cases using two filters can help provide movement through out the tank eliminating dead spots and give you enough filtration for even the largest systems.  You would not need to buy two large filters, but rather two smaller sized filters that their total tank ratings combined are large enough for your system.
8. Many times people will buy a filter that is oversized for their tanks and feel this will help decrease the normal maintenance that needs to be done.  But this is not necessarily true.  While it may extend the time between maintenance, no matter what type of filtration system you employ on your tank you will still need to do monthly water changes, algae scrapping and normal maintenance on your filter system.  Don’t over size your filter thinking it will make your life easier.
9. Look at the warranty offered by the manufacturer and look for reviews on their filters.  Many times if a manufacturer is offering no warranty or only a short warranty this might be a sign that the filter is not meant to last long.  If they are offering a 1+ year warranty more than likely the system will be around for quite some time for you.
10. Research the type of fish you are planning on keeping and buy a filter to suit their needs.  Goldfish and South American Cichlids tend to be very messy fish compared to peaceful tropical fish such as tetras, guppies and barbs.  Saltwater fish are very sensitive to water quality and need a very good biological filtration system to make sure their waste is broken down very quickly.