Biological filtration refers to the system of bacteria and other microorganisms living in your tank. This system does not live in your tap water and must be allowed for form by a process called "cycling" which concerns the conversion of Ammonia(waste) into NO2 and then into NO3 which are less toxic substances. cycling is talked about more in section 4.
Mechanical filtration happens when water is passed through some type of porous substance to catch particles in the water. This is the most common form of filtration. There are a few different types of aquarium filters but all of them have mechanical filters. Most filters need to have the porous filter bags cleaned or replaced every 2 to 3 months. More advanced filters require less maintenance but are usually not used in the 20 to 30 gallon tank range.
Chemical filtration occurs when water is passed through a chemical which absorbs toxins from the water. Carbon is the most common chemical filter. The filter bags found in most filters contains carbon so the filters act as both chemical and mechanical filters at the same time, capturing toxins and waste particles.
i. Power Filters
Power filters come in various different sizes and are relatively inexpensive. These filters have filter bags which much be changed every 3 months and a "bio-wheel" which is supposed to give a home to the bacteria that acts as a chemical filter. Power filters are the most commonly used filter, especially for beginners and smaller aquariums. The main downside to these systems is that they can be loud, even if the box advertises a silent product, most likely the motor will start to hum after a year
ii. Under Gravel Filters
Gravel Filters work by causing the water in the tank to be sucked through the gravel and then pushed through a tube containing carbon and then out into the top of the tank. This is done by placing a grate under the gravel in the aquarium. Then 2 tube are attached to each end of the grade and run from the bottom to the top of the aquarium. The tubes contain a carbon filter and a bubbler which forces water from the bottom of the tube up and out into the top of the tank. This water movement causes the water to be sucked through the gravel. This allows the gravel to act as a mechanical filter. Since the suction traps waste in the gravel, the bacteria of the biological cycle grows in the gravel and allows it to act in the way a "bio-wheel" would work on a power filter. In general these filters are only used on a small scale if ever. While they sound good in theory, they really don't work too well. The gravel accumulates a lot of waste which must be siphoned out more often that with a power filter. The only good thing about the gravel filters is that are very inexpensive. But your really getting what you pay for and I would recommend paying more for something that does a better job.
iii. Canister Filters
Canister filters are the more advanced filter system and are more expensive.< The system works by causing water to be taken out of the tank and then pushed through a canster or some containter which is segmented into different chambers. Each chamber in the canister is a type of filter and once the water is passed through all of the chambers it is pumped back into the aquarium. These filters are a little more technical than the others but once they are set up they do not require as frequent maintenance.
The maintain a livable temperature for your fish, the aquarium may require a heater. There are several types of heaters but the most commonly used is a hanging heater which hangs from the back of the tank with part of the heater submerged. Another common heater for higher end filtration systems is the canister heater or some type of heating device which is incorporated into the filter system. Most heaters can be set to a certain temperature. The temperature sensors on the heaters are usually not accurate and you could end up cooking your fish, so another good investment would be to buy a digital thermometer to give you accurate temperatures. These thermometers cost around $10.00 but are worth the price
Certain fish may require special light but other than have a light source, fish do not have any light specifications. Specialty aquarium light bulbs are needed by the plants which act as shelter and support for your fish. With a better light source your plants will grow faster and your tank will look better while giving your fish more hiding spots.
V. Chemicals Chemicals can be used to change the pH of an aquarium and there are chemical test kits that allow you to track the cycling of your tank as well. There are also products available to help speed up the cycle or help fish and plants to grow. Detoxifiers, pH adjusters, bacteria boosters and other chemicals can easily be found at any fish store.