Mud bed filtration is similar to that of other sand based aquarium filtration systems, however the substrate is made of such a small grain size that it is very much like mud. A good filtration method to equate a mud bed to is the refugium.
In the refugium filtration method various types of macro algae are grown in the fine sand. The mud bed filtration system is effectively the same, however instead of sand mud is used instead. It is not just normal mud though, it is a mud which is full of various elements and minerals. These elements and minerals are slowly released from the mud into the water column.
A mud bed system is normally run in an aquarium or some other container located under the main display aquarium. Some aspect of mechanical filtration is required to remove any large particles from entering into the mud filtration area. This can be performed at the end of the overflow(s) by passing the water through very small chunks of live rock etc. After the water has been through the mechanical filtration it enters the mud filtration aquarium. The macro algae consume nutrients from the water and various elements are slowly released into the water from the mud bed. The water then normally passes through some type of grid or through another chamber full of small pieces of live rock the purpose of which is to prevent any of the macro algae from leaving the mud area and blocking the return pump. Once the water has passed into the pump area it is returned to the display aquarium.
In a mud based system the lights are normally left on for 24 hours a day. This allows the macro algae to photosynthesise and grow. It is only when macro algae grow that they consume nutrients from the water. It is best to use lighting which is designed for the growth of plant life. A couple of fluorescent tubes will suffice, ensure as said that they are for plant life and have a Kelvin rating of around 6000. Lights with this Kelvin rating have more colour in the yellow/red area which benefits the plants.
A normal rule of thumb for a mud based filtration system is to pass in the region of 10 times the total water volume per hour. This needs to be considered when designing the aquarium system as the total amount of water in the system will determine both the overflow size and the size of return pump required.
As with a refugium there may be a requirement to harvest the macro algae if it becomes too dense. Never pull the algae out by the roots – instead cut the plants back with an old pair of scissors or similar. Harvesting the algae will allow more light to penetrate into the areas where the algae grows. One point to remember is that you should not add this macro algae back into the aquarium as you may reintroduce the nutrients back into the water.
A mud based system combined with macro algae removes a lot of the nutrients (nitrate, phosphate etc) and dissolved organic compounds as well as replenishing trace elements.
A lot of aquarists who utilise a mud based system for filtration do not run a protein skimmer. It is not recommended to do so by these aquarists because of the amount of particulate matter which is extracted from the water by protein skimmers. When viewing a mud based system you can actually see the fine particulate matter in the water. The water is still clear, however it is full of fine matter which some life in the aquarium can use for energy.
Of course there are also aquarists who do choose to run a skimmer, however the majority of these run the skimmer part-time (i.e. throughout the night, turned on/off via the use of a timer). The majority of these aquarists decide to run a skimmer as they are very wary of turning it off. I have run a system using a mud based method combined with live rock for many years without problems. The only thing you need to ensure when you run a system like this is that detritus is removed from the main display aquarium regularly and that weekly water changes are performed. A lot of aquarists who do not run a skimmer on their mud based systems do not have a sand bed in the display aquarium due to the build up of detritus which can occur. Instead they go ‘bare bottom’ in the aquarium. The glass which is visible at the bottom of the aquarium quickly becomes covered in coralline algae as well as other types of life so looks more natural as time passes.
The aquarists who decide not to run a skimmer have reported a higher level of particulate matter visible in the aquarium water which the corals, and other filter feeders consume. Because of the amount of particulate matter in the water column you should see good polyp extension from your corals as well as hopefully having success in keeping some of the harder to keep corals. Obviously you cannot just expect to be able to keep these more difficult corals just because you are running a mud based system – you must still ensure that you have optimal water parameters and that the requirements for the livestock in question are met.
Implementing a mud based filtration system is fairly straightforward. It is best to have an aquarium which is split into three or four sections. This can be accomplished yourself using glass and baffles or you can have one made for you at your local fish shop.
The first area is where the live rock is placed and is where the water from the display aquarium, via the overflow(s), enters the filtration aquarium. The purpose of this section is to remove any large detritus from the water as well as break up any air bubbles. Using live rock in this area is a good idea rather than another type of media as it is a natural filtration medium and will actively help in filtering the water rather than hindering it as other types of media could do.
The second section is where the mud is placed as well as the macro algae (caulerpa etc). The mud is poured into this section and the macro algae planted within it. To give the macro algae a chance to put its roots down trap it gently under a piece of live rock. Once the macro algae takes hold the piece of live rock can be removed. It is advisable to add a few varieties of macro algae as some may not take root. Caulerpa is one species which does tend to do well and there are numerous varieties available. Because of the amount of water flow which can be created in the central chamber some people choose to create small containers 1 inch or so high and 1-2 inches apart in the bottom of the chamber. This prevents the mud from moving around and building up in a pile at one end of the chamber.
The third or fourth section is where the return pump is located and is protected from the second central section via a baffle of some type or even more chunks of live rock contained in a chamber. If you are running this mud aquarium as your sump then your heaters for example can also go in this area.
A couple of fluorescent tubes will suffice for the lighting above the mud section and should remain on for 24 hours per day. As said attempt to use tubes which are designed for plant life as this will help the macro algae grow.
Because of the mud releasing essential elements into the water the mud bed will eventually expire, therefore it is recommended that half of the mud bed is replaced about every two years however I would recommend that you rely upon the manufacturers recommendations in this area.