Learn About Aquarium Filtration
The filter is the most important part of your aquarium. Without proper filtration, no fish or plants will survive.
Most people get into the aquarium hobby with no prior knowledge. They usually start with a beginner system with minimal expense and beginner equipment. High maintenance and dead fish are not the way to enjoy the hobby. Unfortunately, that is what fully 60% of beginners get, and they leave their aquarium in the basement or sell it at a garage sale having never really enjoyed the experience.
To prevent this tragedy, an upgrade in the filter unit is all it usually takes. An undergravel filter is just not sufficient for anything bigger than a sparsely populated 10 gallon tank (and a corner filter is not useful for anything larger than one betta fish).
There are a lot of complex ideas, and complex explanations that use big words to "wow" you into thinking someone is an expert, but it all boils down to three types of filtration that are necessary to keep your tank healthy. In layman's terms, it's all very simple.
Mechanical filtration is the easiest. Large particulates such as uneaten food, fish feces, other debris must be removed from the water. The part of the filter that does this can be as simple as the bed of gravel covering your undergravel filter, the filter floss inside the corner filter, the filter floss or foam covering the cartridge in your back-mounted or canister filter, and the pre-filter in your wet/dry system. Before the water goes into any other part of the aquarium it passes through this first layer of debris-trap and the largest particles are stopped from passing further.
Change the floss, vacuum the gravel, change the cartridge, or (in the case of most aquarium canister filters), wash the pre-filter and the particles are discarded without a chance to pollute the tank by decay.
Chemical filtration is the next step. When the water comes from the tap, it is already toxic to fish. Most filters contain a layer of carbon that takes out the chlorine and early-stage ammonia in the water by actually absorbing it into the carbon. When the carbon has absorbed to its maximum ability, the carbon or cartridge containing it is discarded and thus, dissolved organic compounds are discarded as well. Put in a new cartridge or (in the case of most aquarium canister filters) new chemical filtration (carbon) media and the problem is solved.
Proteins and amino acids (more dissolved organic compounds) also build up over time. Protein skimmers are useful for getting rid of these and other chemical pollutants before they turn to algae-producing nitrates. This is indispensible for saltwater tanks and very beneficial for freshwater tanks as well.
Most portein skimmers create a powerful, spiraling column of air that churns the water creating bubbles that collect organic waste, zoom to the top and "pop" in a collection cup where the waste is captured then discarded (you might have seen this type of "foam" in a dirty hot tub).
More about Protein Skimmers will be discussed at another time.
Biological Filtration: This is the one most overlooked by the beginner.
Biological filtration is mostly concerned with the waste products of ammonia (NH4+) and nitrite - two biological by-products from fish metabolism and decay in the tank. When too much ammonia builds up, the fish cannot breathe. They stress. They die.
Zeolites can be added to the filtration media to help change NH4+ to NH3 (a less toxic form of ammonia). However, using this method is not always successful (never in saltwater aquariums), and it requires consant vigilance. Frequent changing of the zeolite is necessary so that it does not "oversaturate" with ammonia and then just "spill" it back into the tank in one big disaster of highly ammoniated water and sick or dead fish.
Good Bacteria & BAD Bacteria
Most biological filters use "good" bacteria that thrives on NH4+ molecules as a food source. Like Packman, one type of bacteria "eats" then converts ammonia to nitrite. Another naturally occurring bacteria converts the nitrite into a far less harmful form of nitrogen waste: nitrate.
Within two weeks of adding fish to a new setup, these "good" bacteria are usually well established in aquarium gravel, on tank walls and on plant surfaces (although there are never enough on those surfaces to support an actual tank of fish). You can also add them from commercial prepartions sold for the purpose of populating your tank with healthy bacteria organisms to get a good headstart.
Both of the bacterias thrive and multiply in an environment with plenty of oxygen and plenty of biological waste. They live in "bacteria beds" where you encourage them to grow strong and healthy by giving them a large surface area to call home and feeding them plenty of organic by-products to digest. The "bacteria bed" of most aquarium canister filters is in a chamber holding bio-media, ceramic rings, bio-stars, bio-balls or some other type of media made to house bacteria cultures.
Given enough room to hold on, the good bacteria multiply as your tank grows and produces waste. In a perfect aquarium world, the baceria and the waste exist in perfect balance: Less waste=less bacteria. More waste=more bacteria.
In a tank with an undergravel filter, the gravel supports aerobic (good) bacteria. As the water passes through, the bacteria digest the ammonia or convert the nitrite and the water is clarified. The same action happens (only much more efficiently) in a tank-mounted power filter with a bio-wheel or in a canister filter in the biological media chamber housing bio balls or bio ceramic. Biological filtration at its best happens in a wet/dry filter where the mass and surface area of biological media can be enlarged as much as you need.