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    An External Biofilter

FAQ - Read the frequently asked questions about "Filtration".

An efficient biofilter is necessary in African chichlid breeding. Although it is true that quality equipments can be easily bought in the pet shops but are often expensive. However, their main disadvantage is the low volume of the filtration media. That is the reason why to make the filter by your own idea and need. It is rather an easy job and also cheaper than the allowable commercial canister filters. The only requirement if we want to have the filter under the aquarium (i.e. in the cabinet), is the hole in the aquarium through which the water is drained into the filter. The overflow box is needed when the tank is not drilled. I recommend to drill the hole before the tank is glued because drilling the finished tank is much difficult and often impossible due to the glass crack risk. There are many options where to place the filter, for example next to the aquarium in order to level the water in the aquarium and filter. I have decided to place the filter under the tank from aesthetic and spatial reasons. It required the drilled tank. Read more about the overflow solution.

akvárium Let’s continue with the filter building. First, we need some glass aquarium, i.e. older one. I had an empty unused old 60 L tank (Pic. 1). The most important thing is to think how many chambers should the filter contain. The whole filter design with dividers and measurements is pictured below (Pic. 2).

Pic. 1 - An original old tank

celková koncepcia filtra

I have chosen five chambers design. It was needed to cut the glass dividers with one dimension same like the deep of the filter and the second one (high) was cut in accordance with the chamber. Three glass tables have the same dimensions and end 2 cm under the top edge of the filter. These tables are also glued 2 cm above the filter bottom. The water flows under these dividers into the individual chambers (the yellow arrows). The first four chambers are 10 cm wide. Between the chamber 2 and 3 and chamber 3 and 4, there are the next narrow chambers (2 cm wide) which serve for the flowing water aeration either by the water gravity over the divider or by adding the air stones into both narrow chambers. An air is flowing against the falling water and then the aeration is much effective. An oxygen is important for the bacteria living in the filter media. The bacteria consume the oxygen so that they would be not able to live in the last chambers because of the oxygen deficiency. That is the reason I aerate also the water ahead of the chamber 3 and 4.

If the glasses are cut, we can glue them into the filter by the silicone glue. The most difficult part of the work is to glue the dividers which are only 2 cm close to each other. You need the finger like E.T. :-) No funcional properties are affected but it looks unaesthetic. A special tip for hair-splitters: use the transparent silicone ;-)

Obr. 3 - Vlepenie prepážok Obr. 4 – Vlepenie prepážok Obr. 5 – Pohľad zhora The dividers 1, 3 and 5 (Pic. 2) should be glued 2 cm above the bottom to allow water to flow under the dividers. It is a good idea to brace the dividers by 2 cm thick wood brace while gluing (Pic. 3 and 4). Fix the glasses by an adhesive tape and allow the silicone to cure for 24 hours. After the dividers are glued, it is useful to glue the longitudinal braces under the up edge of the filter (like seen on the aquariums) (Pic. 5). In case of the filter, these braces serve for placing the cover glasses to prevent the water evaporation from the filter.

Obr. 6 - Filter - vata Obr. 7 – Filter - pena Obr. 8 – Filter - čerpadlo
After the glue is dry, we can insert the filtration media, return pump, air stones, connect the hoses and the filter can be cycled. The first chamber serves for retaining the bigger particles floating in water. I use to use the Hagen filter cotton in the chamber 1. The retained sludge in the white cotton is pictured (Pic. 6). In the chamber 2, there is a Filtren TM 20 foam and in chambers 3 and 4 the denser TM 30 foam is used. Pic. 7 shows the finer filter foam in chamber 2 and denser one in the chamber 3. You can see the air bubbles flowing from the air stone which is placed between the chamber 2 and 3. Chamber 5 contains the return pump and heater (Pic. 8).

Obr. 9 - Celkový pohľad Obr. 10 – Filter v prevádzke Now, the filter is ready (Pic. 9). Don’t forget that the filter is cycled approximately after 3 weeks (Pic. 10) when the bacteria converting the ammonia to nitrite and nitrite to nitrate are multiplied. The latest must be removed by the regular water change from the aquarium. ;)

FAQ - Read the frequently asked questions about "Filtration".

DISCLAIMER: By building this DIY project you agree not to hold the author responsible for any injury or bodily harm you may cause to yourself or others. Read all safety instructions pertaining to equipment prior to use.

Copyright © 2003 - 2017 Robert Toman

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Description of my aquariums, breeding history, photos and everything that happened in each tank during the time.


Tips and tricks for doing a various tools and equipment for fish keeping. Here, you will find an inspiration for your own attempts to simplify the fish keeping.


My own experience with African cichlids breeding. A detailed description of fish keeping and breeding with special things I found during the years of fish keeping.



My own photos anf videos of fish kept in my tanks.

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