Major constituents of a typical aquarium are the filters. There are three types of filtration processes all which should be applied in your own aquarium: mechanical, chemical, and biological. The most common filters used these days for fish only systems are wet/dry filters in addition to heavy duty protein skimmers. I don't recommend getting an under gravel filter these days because wet/dry filters do most of the under gravel filters work with more efficiency. Also it makes things more difficult when you have clean out your gravel by siphoning because the under gravel covering gets in the way. Canister filters are another option that is also a good choice typically seen in fish only systems. The canister filter is basically a canister which holds several types of filtration media. Liverock is an extremely good quality of biological filtration for one's aquarium setup especially reef aquariums supporting corals. The bacteria on the live rock eat nitrate and convert nitrite in the water making your water more clean and suitable for your fish. For the wet/dry filter, you have the choice to either get an internal one, which is built inside your tank or you can get an external one which can be placed underneath your tank (more common). An external one may be better for you if you want more space in your tank. If you get an internal one your tank space is taken up by about 1/3 depending on how large the filter component is.

    Undergravel filters were good filters back in the day when it first easily solved the problem of biological filtration and breaking surface tension to allow oxygen in the water. If you have trouble siphoning out your gravel then I do not recommend getting one of these because they will trap tons of detritus underneath the filter trays. If you have an invertebrate tank which mostly has inverts then the use of an undergravel is useless because modern filtration methods replace all purposes for an undergravel filter which just gets in the way. Undergravel filters should mostly be used for freshwater fish tanks that house goldfish and other cheap fish. With this filter there is usually a tray and an air tube for the air to get to the bottom of the tank. The fish then don't have to swim to the top of the tank to get air they can just absorb oxygen right out of the water column. Now a days, most marine aquarists tend to not even bother with undergravel filtration.

    Canister filters are just what they sound like, basically a canister with several types of filtration media in it fed by hoses to circulate system water. Carbon, gravel or rocks, and some sponges consist of filter media in order to catch debris, clear the water, and break down fish waste. This filter usually fits underneath your aquarium stand. This kind of filter should be mostly used for small tanks anywhere from 20-60 gallon tanks. Anything bigger should not have a canister filter usually because they aren't large enough to efficiently handle huge tanks. Canister filters have many valves and tubes for transporting filtered or unfiltered water which may be a bit confusing for the beginner. Some canisters come with a trickle tray to place on the top of your tank for aeration and other types may not. Most aquarists tend to like this type of filter for medicating tanks because they are simple setups. In general these are good filters to use for the average size tank.

    Wet/dry filters are probably the most popular types of filters used for marine fish only tanks. Internal wet/dry systems contain many chambers where the water is treated in different ways and then fed back into the tank by a pump. These chambers contain sponges for collecting the debris along with some carbon to clear the water and remove odors. Next there is usually a slot for sticking the heater in and some room specially made for a protein skimmer to be installed. The main section of the wet/dry is the place in which biological media (i.e. blue bio-balls) are kept. These bio-balls are for important bacterial colonies to grow on. These balls should not be cleaned off because they contain bacteria that breaks down ammonia and nitrite. As the water trickles through the air and bio-balls, the bacteria reacts with the ammonia and oxygen hence the name (wet/dry) filter. Wet/dry filters also interact with plenty of oxygen for the fish and inverts to consume. The overall pH level can remain constant even throughout the night when the tank tends to have low oxygen. Now for the external wet/dry setups they are usually under the tank in the stand, which carries out the same duties as an internal would. However, internal wet/dry setups take up lot of space so if you want more tank space for the fish then I would recommend getting an external we/dry for use beneath the tank.

    Protein skimmers, also known as foam fractionators, are essential for all marine tanks because they take out hard to filter "crud/foam". Protein skimmers generally are long cylinders that have an air pump attached to it, the micro air bubbles force the unfiltered organics up through the cylinder to the top into a collection cup. From there you just have to empty the waste from the cup each day or week depending on how you adjust the consistency of micro bubbles. You can adjust the air flow with the air valve of the pump in order to get the right consistency of foam which should be kind of thick and not too watery. With a protein skimmer you will be amazed at how much it clears the water up. For reef tanks, powerful protein skimming is an necessity. If you notice your skimmer is producing more waste than normal, you might need to siphon detritus from the tank and clean out your filter pads. A water change will also benefit the tank.

    Live rock is an excellent and commonly preferred source of biological filtration. Reef keepers tend to use lots of liverock, so if you want a fish only tank, liverock would not be a necessity like it is for reefs. The rocks contains important bacteria and organisms which break down fish and invert waste to non-lethal levels. When starting a reef, it would be wise to introduce live rock as your foundation. The use of only live rock and a protein skimmer for filtration is called the Berlin Method. Live rock can also be used for cycling a tank at a much faster rate. I would recommend getting plenty of live rock if you are just starting out your tank or cycling your tank. There isn't much you have to do for live rock and it doesn't need intense lighting to stay alive. One draw back of live rock is that some unwanted critters come with the rock like mantis shrimp and bristle worms. They can cause destruction for your reef by eating corals. I therefore advise to you when you start out with live rock, that you place a small enclosed container with poked holes and bait inside. This container will act as a trap for the unwanted critters. Place the container around different areas and within a week you can eliminate these critters. You should keep about 2-3lbs. of liverock per gallon. "Un-cured" live rocks means that there are decaying organics still on the rock which is also considered "die-off". Those die-off will cause the ammonia levels in the tank to rise, but since half of the rock is still alive, the alive part of the rock will cure the dead organics from biological processes. When you purchase the liverocks, let them sit in your tank for about 2 weeks to neutralize the waste levels, that is only if you rocks are not yet cured.

    In reef tanks and some fish tanks, water circulation is a necessity. The currents in the tank carry away carbon dioxide and bring oxygen and essential minerals to the corals. It carries away any waste that the corals produce. If there are stale or non moving currents in the tank, then the corals will not be very happy because they like consistent and stable conditions. Each coral will require different current depending on where they came from. Mushrooms for example do not like the current that much however pulsing xenia and star polyps love to take advantage of the current. With more circulation, the water is turned over more and the oxygen and carbon dioxide can enter and exit the water much easier providing a better environment. The total water volume should be turned over at least 5 times per hour. Try to avoid a whirl pool effect in the aquarium because corals appreciate random flow. Make sure there is turbulence (gives variety), and wavelike motion, where the water moves back and forth in the tank. There are wave-makers that have this effect. During the night, the corals enjoy resting a bit so keep the circulation pumps calm for a few hours, also during feeding time the pumps can be turned off for about 5-10 minutes. Sufficient circulation will take away cold and warm spots in the tank. I find wave-making devices to be successful depending on how well you position your powerheads throughout the tank. It also helps to place circulation throughout the back of the tank so that water is flowing in ALL places and doesn't allow much time for detritus to settle on the bottom.