Filtration in Planted Aquarium

One of the most important aspects in planted tank is filtration. Many have the wrong impression that filtration in an aquarium is only to screen out particle or debris to make the water clean, but the real function and importance of filtration in a planted tank is perhaps a lot more than that!

Making the water clear by filtering out the suspended impurities, debris or other solid particles are just a small role of filtration in planted tank. To understand the function of filtration in a planted tank, we need to further break it down to mechanical, chemical and biological filtration.

1.   Mechanical Filtration

The simplest and most straight forward filtration of all. This is the part where it makes the water clear by screening out the solid impurities in the tank physically. Normally some sponge or cotton wool in the filter will be able to do the job. While maintaining a clean and clear water is visually very important in this hobby, therefore mechanical filtration is something a planted tank cannot do without, but that is about all, purely for aesthetical reason.

2.   Chemical Filtration

Some of the impurities are not possible to be removed by mechanical filtration, for examples, colors, smells, dissolved substances and other very fine particles. To get rid of these impurities, chemical filtration is necessary. Activated carbon is commonly used as a chemical filtration agent, because it has the ability to absorb colors, smells, dissolved substances and very fine particles. However, there are 2 major disadvantages of chemical filtration (activated carbon):

Due to the above reason, especially reason 2), activated carbon is not usually used as an everyday filter media in normal planted tank. It is mostly used during the initial set up of the tank where there are more impurities and too much excess nutrient exceeding the plants uptakes. Otherwise, activated carbon is normally used as a “remedy” on temporary basis to certain problems such as tanning water, water with unpleasant odors or if there are certain impurities in the tank that cannot be filter out mechanically.

3.   Biological filtration (Nitrogen Cycle)

Arguably the most important type of filtration in planted tank. Biological filtration is referring to the use of microorganism to break down harmful substances (i.e. NH4, NH3) in an aquarium and transform them into inert or even useful substance (NO2/NO3) for the flora and fauna. Microorganism used in biological filtration also called beneficial bacterial and it needs to be cultured. There are 2 types of bacteria playing part in converting NH4/NH3 to NO2/NO3. The most important type which does most work is the aerobic nitrifying bacteria. The second type is heterotrophic bacteria which is not as effective but still have the ability to convert NH4 to NO3. However, heterotrophic bacteria also capable of converting NO3 back to NH4 in certain conditions. “Seeds” of these beneficial bacterial can be introduced into the aquarium in dry (power) or liquid forms which are both commercially available however most of the commercially available bacterial agent are the heterotrophic type. These “seeds” contain millions of microorganism which will colonize and multiply in the filter media or in the soil and provide the filtering function. We can also introduce bacteria seeds to a new tank by transferring some aged water from an old/existing tank or taking some sludge from a running filter which contain the bacterial colony.

Unlike mechanical or chemical filtration where the efficiency decreases with time and needs to be changed or replaced, biological filtration actually gets better with time. A matured bacterial colony will perform the filtration function more efficient then a new colony, simply due to the size of the colony (matured colony = more bacteria). Therefore, it is of critically important not to destroy any biological filtration media throughout the life time of an active planted aquarium, which can happen unintentionally during the washing of filter. Chlorine inside any “new water” used during the washing process of a filter may potentially kill the bacterial colony, so it is important to use the aged tank water and not the tab water for this purpose.

Filter media does not need to be totally “clean” as the cleaning process will unavoidably flush away some beneficial bacterial. Therefore, just make sure you rinse the filter media lightly with aquarium water to get rid of any debris or dirt but not to ‘wash” the filter media vigorously, even with aquarium water. Beneficial bacterial can be replenished from time to time by adding the “seeds” or bacterial agent into the filter media or directly into the aquarium. To achieve more greater biological filtration efficiency, we need to provide large surface area in the filter media for 2 reason:

Therefore, selecting filter media for biological filtration purpose is important - filter media that offer bigger surface area is preferred.  

Nitrogen Cycle diagram  

Important criteria in selecting a filter for planted tank

1.   Flow rate

For freshwater fish keeping tank, a filter that has a flow rate of 2-3 times tank water volume is enough to provide good filtration. However in planted tank, flow rate of filter become one of the important factors. Plants do not have the ability to move and grab all those nutrient in the water column. Thus we need to deliver it to them. Hence higher flow rate is necessary to keep up with the plant uptake.

Flow rate of a filter in a planted tank should be at least 3-4 times more than the total volume of the aquarium, i.e., if you have a 100L aquarium, the flow rate of the filter selected must be at least 300-400 L/hr. This means that the filter is able to turn over the water in the aquarium 3 to 4 times per hour. Theoretically, you should select the highest possible flow rate without creating too much turbulent in your tank that will upset your flora and fauna.

2.   Filtration volume

Filtration volume simply means how much filter media can be stored in the filter. As mentioned earlier, we need the largest surface area out of the filter media. Beside selecting a good filter media with large surface area, providing more filter media in volume also serve the same purpose. Therefore, we need a filter that can take as much filter media as possible. In this aspect, canister filter is in general better than other type of filter due to large compartment for filter media.

Beside this 2 important factor in selecting a filter for planted tank, other generic factor such as durability, spare part, noise, ease of use, etc will not be discussed here.

Types of filter for planted aquarium

Comment from Josh
I am a filter guy. I always prefer “over-filtration” – if there is such term. In most of my tanks (3ft tanks), I run with 2 filters for a few reasons: