Media Retainers and Fish Cages

Media Retainers and Fish Cages

Moving bed filters offer various solutions for the problem of retaining the moving media in a place without restricting the flow and preventing loss of media. At a high flow rate, a simple grid at the exit point is often not good enough to avoid this from happening. A retaining grid that meets the requirement of a specific media is need. We approach the problem in a slightly different way.

One is often oblivious of the fact that the media are clogging around the exit point in an immobile state. This prevents maximum nitrification because of oxygen depletion between the media elements. It also restricts the flow resulting in an under performing system.

In the moving bed vessel, we either have a pump-fed or gravity-fed arrangement. In the first instance we then notice the filter overflowing and we do some workaround to solve it.

In the second instance the flow rate is restricted, and we also have to make a plan. In any case, things did not work out as we initially thought or calculated. The head between the filter chamber input and output becomes too much.

We use various media restrains, each depending on factors like the type, media volume and flow rate required, but here we describe our favourite using our Retainer Net Bin for systems with a flow rate up to about 40 m3/hour. And you can always use them in parallel to increase the flow rate.

Mutag Bin


Fig 1 – Retainer Net Bin

With an effective volume of about 180 litre, each Net Bin can be loaded with up to 90 litre media (at a 50% loading).

Using 90 litre of Mutag BioChip media, the filter will remove all the ammonia created by feeding 1.25kg of 35% feed per day. That is feeding an astonishing 37kg/month. Quite a collection. By comparison, using a K1-type media you will need 360 litre media, or four net bins for the same job.

This retainer can be used in various configurations as are illustrated below in the three figures. The orientation will determine the maximum flow rate that can be attained, and I am sure you can think of others:


Net Bin

 With this end-on arrangement we sustain ten koi averaging 65cm in size with only 24 liters (two buckets) of Mutag BioChip. We are still testing the feed load. (March 2020 still maintaining the same fish from same client)

 net bin

 In the arrangement the chamber is typically built and the net cage positioned such that water flows out through three sides.

net bin

Below a video of a moving bed filter in an end-on configuration using a plastic bin and 24 litre of Mutag Biochip supporting a collection of large fish.


The Net Bin can also be used as a Net Cage for holding fish. Any fish farmer can find a niche for this robust net retainers.

 Servaas de Kock
May 2018

(Update: March 2020 still maintaining the same fish from same client. Two systems.

What we did learn is the importance of keeping the pH correct in such a high feed rate. If the pH drops below 5.5 the organisms growth on the media is impacted and ammonia increases. StabHi pH Control Pebbles and Powder is important to maintain pH)