If you’re new to fish, you may have wondered what kind of filter you should use in your aquarium care. Many brands of filter have a printed number on them that is supposed to indicate what size of aquarium it’s good for, but this number is only correct some of the time as each type of fish and various stocking levels have different filtration needs. The general rule of thumb is that the filtration should allow for the entire water volume of the aquarium to cycle through the filter four times every hour (the turnover rate); however, a number of factors can affect this number.
First, take into account the number of fish you have and the size of the aquarium. For fish under 6" you will want approximately one gallon per inch of the adult size of the fish, over 6" the fish need a tank that is at least four times as long and twice as wide as the fish. It is possible to keep more than the recommended stocking level in the aquarium, though it will require a lot more work to keep the water in excellent condition and to ensure that no issues arise due to the restricted space. If you have a higher-than-recommended stocking level, additional filtration will be needed to compensate, often upping it to a turnover rate of 6-8 times every hour. In some cases, water maintenance products such as EcoBio-Stone can aid in keeping the water within acceptable parameters if the fish are overstocked, though this should be avoided as the fish also need individual swim space and sufficient oxygen for all inhabitants.
Special space and filtration requirements apply to goldfish, cichlids and many large carnivorous fish. These fish still need the same kind of space recommended for their size, but in general need a turnover rate of ten times every hour to provide sufficient oxygen exchange and mechanical removal of waste and uneaten food. These fish have larger stomachs than the average tropical community fish and so produce much more waste. Bear in mind that goldfish and fish such as oscars and piranhas reach an adult size of a foot or more so be sure to plan accordingly; it's a particularly nasty myth that these fish grow to the size of their containment and no larger. While growth hormones will stop them from growing when there is no more room to do so, the fish will generally have shorter life spans and can have many other health issues; oscars and gouramis, amongst others, will only experience a cessation in the growth of their head while the body still grows -- eventually, their mouths will morph too much for them to be able to eat.
In many cases, the type of filtration must also be considered. My favorite filters are HOB filters that hang on the back of the tank and remove waste while facilitating oxygen exchange by keeping the surface of the water moving; however, HOB filters do not work for all fish. Because of the current produced by the workings of the filter it is not the ideal choice for weak swimmers such as bettas and smaller angelfish, while its open intake makes it dangerous for tiny creatures such as fry and shrimp. For these fish, sponge filters or undergravel filters coupled with good aeration may be the best option.
There are some measures that can be taken to lighten the filtration burden in your aquarium. The best thing for any aquarium is to have healthy inhabitants and for that you need the best possible water quality, meaning timely partial water changes as well as a strong biological filter. Biological fitration is achieved by having healthy amounts of bacteria in the aquarium and can be helped along by commercial water maintenance products such as EcoBio-Block. EcoBio-Block not only helps by adding good bacteria to the water, it also provides the essential minerals that are used up by fish and would otherwise have to be replenished through partial water changes alone.