Filters that purify the water in the aquarium come in different forms and have different functions. The most common types of aquarium filters are the hang on back filter, the canister filter and the under gravel filter. Under gravel filters use the substrate as a filter medium. Not only doplants produce the oxygen that the fish and invertebrates require, they also act as a natural water purifier. That is, the plants will absorb harmful substances like ammonia and nitrogen from the fish and invertebrate waste. These are some of the reasons I always say, "A tank with healthy plants is a tank with healthy fish."
Let's consider filters from the standpoint of the three basic essential requirements of plants: water, light and carbon dioxide. Aquatic plants depend on the water for life. The fine aquaplants that aquarists cultivate, however, survive in a nutrient-poor environment. When the water becomes too rich in nutrients, lower plants life form like algae will begin to thrive and choke out the higher life form : your aqautic plants. Even the best filters available will not be able to purify the water and eliminate this problem. It is inevitable aquarists will have to regularly change the water as a secondary purification measure.
If the filteration capacity of the filter is insufficient, no amount of water changing will be able to prevent algae from invading your aquarium. The fish in your aquarium will also be more susceptible to diseases if your filter is undersized or poorly maintained. The bacteria and microbes in healthy filters will combat pathogens and algae, making it difficult for them to thrive.
Make sure that whatever filtration system you use does not block any light from reaching the plants.
Finally, let's discuss on the issue of CO2. Filters that aerate or churn up the water will cause the CO2 to vaporize. It is like shaking up a can of soda. All the CO2 will go hissing out into the air, making the soda flat. Plants don't like flat soda anymore than we do. Air lift filters, pipe tube, shower, and other concentrated filters, and any filter that cause the water to come into direct contact with the air, will causes the water to lose CO2. Water can absorb 70 times more CO2 than air can, but it also escapes much more easily. These filters are fine for aquaria that the primary concern is fish, but aquaria designed for plants cannot use filters that create CO2-dispersing turbulence nor allow direct contact between the water and air.
The filter that passes in all three areas is the one specially designed for raising aquatic plants: the self-contained canister power filter. But even a power filter cannot compensate for poor practices like shower-style draining that creates turbulence. The position of drainage tube must be located below the surface, hence, you can forget about installing a spray bar.
The power filter can be misused. The method of under gravel filteration with the discharge suck up from below the substrate has some detrimental effect. Firstly, substrate fertilizer cannot be used. Secondly, once the bottom sand start to age, it will no longer be efficient as a filter media. Finally, as the plants grow, their roots will interfere with the flow of water through the substrate. Because of this, the undergravel filter is actually effective only in the early stages of the planted aquarium. It brings oxygenated water directly to the plants' roots and evens out the temperature difference between substrate and water. Though this improves growth in the early stages, in fact it only brings the aquarium closer to the problems that will develop later. Of course, this principle does not apply to the aquarist who changes the layout frequently.
PHYSICAL AND CHEMICAL FILTERATION
Filters can be further divided into two category - physical filters and biological filters. Physical filters use activated charcoal or zeolite to remove ammonia and nitrogen from the water. The common biological filters use micro-organisms that nitrify ammonia and nitrogen, that is, change them into less harmful nitrates through oxidation. Both methods have their advantages, and in many plant aquaria, both are used simultaneously.
In the early stages, until the nitrobacteria have fully developed in the filter media, physical filtration with activated charcoal can be relied upon. But rather than using only the charcoal, putting a bottom layer of a rough, large granule filter material will not only catch large particles of waste but at the same time feed the nitrobacteria in preparation for 100% biological filtration. Activated charcoal loses its filtering ability in one or two weeks. But when that time, effective micro-organisms will be flourishing on the charcoal.
Now, it is very important to understand the timing of the change from physical to biological filtration. The charcoal should not be immediately removed in order to convert to biological filtration, because the nitrobacteria on the rough lower layer will not be strong enough to carry the filtration burden of the entire aquarium alone, in balance will be upset if this happen, and fish will begin to die and algae will sprout up. The bacteria in both areas have been doing its work in filtering up to this point, so that is the amount that is needed. Therefore, the charcoal could continue to be used, but as the material for biological filtration.
One disadvantage of charcoal is that it is a very fine material that needs to be cleaned occasionally to prevent clogs. So it is usually replaced with a material that is just the right size and good for bacterial growth. There is no hard and fast rule for when to change materials, and judging the timing is difficult. Generally, the activated charcoal should be replaced at the time of the first or second clogging. Whether the charcoal is going to be cleaned or replaced, the bottom layer must not be cleaned until the whole filter is switched to biological filtration.
It is no exaggeration to say that the condition of an aquarium depends very much on the performance of its biological filter. When the filter's micro-organisms are thriving, the water will be crystal clear and there is no algae growth.
The chemical reaction that expresses the oxidation process carried out by the nitrobacteria which converts harmful ammonia into harmless nitrate is NH3; NO2; NO3. The bacteria that converts ammonia (NH3) into nitrite (NO2) is called Nitrosomonas, and the bacteria that converts that into nitrate (NO3) is called Nitrobactor. Research shown that the remaining nitrate is about 70 times less toxic than nitrite, but if enough accumulates in the water it canstill be harmful. Therefore, it is always necessary to frequently change the aquarium water even when using a top-of-the-line filter. A filter that can completely dispose of nitrates has yet been built, maybe in the next millenium.
To determine the nitrate and nitrite levels of the water, there are both analog data meters and chemical reagents. In term of cost, the latter will be a better bet though it may not be so convenient. A good estimate of these nitrate levels can be made from the pH value as well. As the level of nitrates increases, the pH decreases, and water that is high in nitrites will have a higher pH. If the pH value hits 5.0, then it's likely that the nitrate level is high.
The pH is affected by two elements. The first element are nitrates, in which they themselves acids. The second element is when nitrobacteria oxidize organic materials, they will consume oxygen and release CO2. Similarly, the level of pollution in a river is expressed as a figure called the B.O.D. (biochemical oxygen demand). This shows how much oxygen is used by nitrobacteria in the oxidation of organic matter, and therefore is an indicator of the level of organic waste in the river.