Rational filling of the biofilter.
We will be talking about a certain “average” freshwater aquarium with some plants or none at all, i.e., the focus is mainly on fish, there is no pronounced overpopulation of fish, and the biofilter is used as the sole filter, meaning its role is tangible not only in biological but also in mechanical filtration of aquarium water. At the same time, the biofilter is classically used without a pre-filter.
Most well-known filter manufacturers initially (e.g., in the manual) provide a recommended filling scheme, but sometimes such a scheme may raise a number of comments. Biofilters, as a component of aquarium equipment, can fall into our hands: Fully filled. Classically, these are: • ceramic rings; • sponges of varying porosity; • substrate for bacterial colonies; • highly porous granulated carbon; • filter floss (synthetic padding). Partially filled, i.e., the filter contains sponges and filter floss. The third option is empty. In this case, in my opinion, the second option is the most successful for the consumer. It solves the problem of adequately sized sponges. Sponges must completely cover the size of the basket, if any, or the entire area through which water can flow at the sponge’s location, and at the same time, it should not be larger in area so as not to take up volume intended for other media.
So how exactly should filter media be arranged in a biofilter?
First of all, one must take into account which side water enters the filter from and which side, relative to the filter material, it leaves the filter. For example, in most external canister filters, water begins its journey through the filter materials at the bottom and ends at the top. While in many internal biofilters, water intake can be from both the bottom and the top, with discharge occurring at the top. Regardless of the presence or absence of media baskets in the filter, I recommend filling the filter as follows:
At the point where water enters the filter media compartments, “ceramic” rings should be placed. The primary purpose of their application in the filter is to branch the flows and evenly distribute the water current across the entire cross-sectional area of the filter material. The fact is that water tries to follow the path of least resistance, i.e., it is easiest for water to start moving from the point where it hits the filter material directly to the intake tube of the filter head.
Thus, in an external filter (with baskets), the water movement will create a kind of funnel, wide (but hollow inside) at the water entry point (bottom) and narrow to the size of the intake tube at the exit point (top), which will practically disable a significant part of the filter materials located in the part closest to the exit (top). In the case of an internal biofilter (or an external one without baskets), water will move along S-shaped curves of the structure (or diagonally from the inlet to the outlet in an external one), bypassing a significant area of all media in the filter.
Naturally, as the media becomes clogged with mechanical particles, the direction of the water current will change, but using the entire area of the media from the start (without waiting for mechanical clogging) is more justified and rational. In an external filter not equipped with baskets, this media does not necessarily need to be placed in a mesh bag; it can simply be poured onto the bottom of the filter, while in an internal biofilter, it is better to use a mesh bag. With a bag, it will be much easier for you to remove the media for rinsing. It is also important to note that ceramic rings should be placed in the filter randomly, under no circumstances trying to lay them out in neat rows, because when laid in rows, they will not be able to break up the water flow.
Following the rings, sponges designed for mechanical filtration should be placed. First (closer to the water inlet), a coarse-pore sponge should be placed, followed by a fine-pore one. Sponges can and should be rinsed regularly. The frequency of rinsing (as well as filter maintenance frequency) should be determined individually in each case; there can be no rigid framework. One of the subjective signals to open the filter can be a noticeable decrease in the intensity of the water current at its outlet. Sponges should be replaced with new ones only as they physically wear out.
Next, you should place the substrate for nitrifying bacterial colonies. One of the most modern such media is foamed glass media, and other media can also be found that can be used for this purpose. When choosing such media, the surface area useful for bacterial colonization should be taken into account. Also, pay attention to the fact that the media should not crumble into powder (this also applies to ceramic rings). Such media must be maximally protected from mechanical dirt getting onto it. This media should be rinsed as rarely as possible, exclusively in aquarium water obtained during water changes. This media should be replaced as it degrades (usually about 2 years), and it should not be replaced all at once, but gradually.
Next, even closer to the outlet opening, various sorbents (activated carbon, ion-exchange resins), peat, etc., should be placed. I will say a separate word about carbon in the aquarium. Its constant presence in the filter is not always justified. First of all, it is necessary for removing medications, as well as at the aquarium setup stage. The constant presence of carbon in the filter is completely optional, and in an aquarium with live plants, it can even be harmful. Of course, you can keep carbon as a precaution, but you should remember that carbon “lives” for about 4-6 weeks and then requires replacement, as it ceases to be effective. Calculating the amount of carbon is quite simple: 1 gram of carbon should account for every 1 liter of fresh aquarium water.
And finally, right before the outlet intake opening (i.e., before the filter pump), filter floss should be placed. It will protect the impeller from dirt particles getting into it. This medium should be replaced as it gets dirty (it can only be rinsed a few times, it is extremely fragile), but you shouldn’t be afraid of this, as this medium is inexpensive. It is important to note that any media planned to be installed in the filter should be rinsed under running tap water before installation.
A biofilter is not a means to keep a larger number of fish, although it does allow for this, but rather an opportunity to provide them with more comfortable living conditions, increase the overall stability of the balance in the aquarium, and allow for slightly less frequent water changes or smaller volumes than without a biofilter.
No matter what the filter looks like, it cannot replace regular partial water changes.
Features of the external filter. The inner world of the external filter. 
