Central Filter

Central Filter


How to build your own biological filter.
By: W. A. Soestbergen
Translated by: Rob Charite


Personally I'm in favor of using biological filtering. Maybe I will make a special page about this topic. But now I want to give you some insight about how to build your own. It was the first time for me and I have to say it isn't all that bad.

It is very important that you know what you are doing before you start all of this, therefore, make a drawing with all the sizes in it so you won't make mistakes. If you have doubts you can (after you have told what you are building) let the glass retailer check your figures before you buy the glass. I choose to use a glass thickness of 8 mm instead of 6 mm (6mm should be enough) but, better safe than sorry !!

The biological filter I tend to make has the following sizes:

  • length 60 cm
  • width  45 cm
  • height 35 cm

The length and the height could not be bigger because of the available space I have to place the filter. The height is chosen in such a way that incase of a power outage the filter won't overflow. People who know a lot about these things say that a filter mass height of 15 cm is enough. It is no use to make it any higher.

What I did:

First I calculate the amount of water that floats out of my tank in case of a power failure.
I use the formula : length x width x height

In my case it is : length 100 cm x width 50 cm x height overflow 2 cm = 10 liter.
That 10 liter is needed for the filter as a extra storage capacity otherwise in case of a electricity failure your filter will overflow and you'll get wet feet.

The length and the width sizes of the filter are known so I only have to determine the height and I did that like this :
I calculated 10 liter that shall overflow in case of a power failure and streams in my filter.

10 liter = length 60 cm x width 45 cm x a height figure calculated later on.
In my case I came out with a height of 4 cm at a capacity of 10.8 liter. As I said earlier; Better safe than sorry."

Conclusion : 15 cm filter mass + the 4 cm for storage capacity in case of a overflow = 19 cm.
On that 19 cm we add 2 cm because it is better to not put your filter media directly on the bottom but to place it 2 cm higher on a small frame, so the water that has to pass trough the filter material spreads at the whole length and width of the segment,
this way we spread the flow speed trough the filter material better.
So we come at a total height of 21 cm.

Because we like to make a stair way effect and we want to integrate a heater I chose a total height of 35 cm.

My biological filter should look like this.


The arrow is the flow direction in the filter.
Segment 1,2,3 and 4 are filter compartments.
Segment 5 is the compartment where we can place the heater.
Segment 6 is the compartment where we place the pump.

The segments between compartment 2/3 and compartment 3/4 we use to place a air stone. The water runs down and the air bubbles go up against the stream. In this way the bubbles have to find their way up against the current and so there will be more air dissolved in the water (this is not everybody's opinion). Others say that it is better to have some plate on top of the overflow that makes a thin film of water and provide the air this way (you can also do this with this design).

Be aware that the last compartment, "Compartment 6", should be big enough to put some sort of pump in! Also the height of the overflow from compartment 5 to compartment 6 (in my filter ) is made in such way that there is about 10 cm of water above the pump.

The yellow lines indicate where the small frames should be.

The sizes of the glass plates we want to buy.
This is the size of the filter on the outside.

  • 58.4 cm x 43.4 cm. amount = 1 (the bottom)
  • 60 cm x 35 cm. amount =2 ( the front and back)
  • 43.4 cm x 35 cm. amount = 2 (both sides )

Now for separating the compartments.

  • 43.4 cm x 30 cm. amount = 4 ( the water runs under these)
  • 43.4 cm x 25 cm. amount = 1 ( the water runs over )
  • 43.4 cm x 23 cm. amount = 1 ( the water runs over )
  • 43.4 cm x 21 cm. amount = 1 ( the water runs over )
  • 43.4 cm x 19 cm. amount = 1 ( the water runs over )

We have to keep in mind that we need some small strips to support the top.

  • 58.4 cm x 2 cm amount = 1
  • 55.0 cm x 2 cm amount = 1
  • 39.0 cm x 2 cm amount = 3

In my case I shall make some kind of pre filter filled with foam cartridges. This way you can clean the cartridges without disturbing the biological filtering. In total we have 18 glass plates which we have to glue together. Don't forget to ask the retailer to sand the sharp edges of the glass. Now we can go to the page (gluing) where I explain some things to get your filter together with some pictures for visual support.

This is the first part. There is also a Glue part.

* Instead of making this filter totally yourself you can also buy a ready made tank and place only the inside glass plates. This will be much cheaper.

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