The Deep Sand Bed – One Of The Most Effective Filtration Methods.
June 23, 2007 · Print This Article
A [tag-tec]deep sand bed[/tag-tec] is probably one of the most common additions to live rock filtration utilised within the marine aquarist world at the moment. The other is called a plenum which I will cover in a future post.
A lot has been discovered over the years in relation to sand beds perhaps the biggest of which is the importance of the sand particle size and the depth of the bed.
Let’s concentrate on the sand particle size first of all.
It is beneficial to utilise sand particles in a deep sand bed which are normally below 0.2mm but above 0.05mm. Even after all the research has been performed it is still unknown as to what the actual ideal sand size is, however the majority of aquarists who run a deep sand bed in their aquarium utilise one which actually feels like silt when touched. There are other aquarists who utilise sand which feels more like mud to the touch.
The importance of particle size was found by aquarists who used under gravel filtration as their primary [tag-ice]aquarium filtration[/tag-ice] method. I will not go into the aspects of under gravel filtration as this is a type which is not used that much any more in the aquarium hobby, however I will quickly go over what was wrong with an under gravel system.
Normally in an under gravel filter substrate such as crushed coral was used. This was because the water had to pass through it easily in order for the biological filtration to function correctly. Crushed coral normally has a particle size of 2 -3 mm which allows water to easily pass through, however it also means that larger detritus particles easily get trapped. This means that unless the aquarist cleans the substrate regularly it will become blocked and the under gravel filter will not run at optimum performance. Worse than performance though is that unless it is cleaned it will block and then start to turn rotten. At this point the biological filtration is failing or failed, and the aquarium inhabitants are threatened by a second danger as hydrogen sulphide will start to form which may destroy any organisms possibly surviving the lost biological filtration.
No matter what type of substrate you use in the aquarium there will be organisms which will make this their home. A great many of these beneficial organisms will not live happily in sand that has a granular size either larger or smaller than they are accustomed to, they will not thrive which will therefore cause a large loss in diversity and population and the sand bed will suffer.
There is another reason that sand based organisms have problems with large grain size and that is that over time the sand grains actually stick together. They can be stuck together in such clumps that the water simply cannot penetrate the bed as a whole which causes dead areas. A while ago it was believed that this was due to calcium build up, however this is now not believed to be the case. It is now believed to be a secretion produced by the bacteria in the bed which causes this effect.
The granular size of 0.05mm to 0.2mm being so small allows the various organisms to be able to move around the sand as they wish without being hindered in any way, therefore they are able to consume detritus contained within the sand as long as there are enough organisms actually present.
There is a common term which is used within the marine aquarist world when it comes to sand beds and that is that the sand bed must get ‘turned over’. What this means is that while the creatures who call the sand bed their home are busy burrowing around looking for food to eat the sand is actually being moved around due to their burrowing activities. Turning over a sand bed is very important is it stops any dead areas from forming, and another important aspect of this movement is that water can penetrate very slowly through the sand bed fostering further bacterial colonisation. It is not unfeasible for a well populated sand bed to be turned over regularly, however again this is dependant upon the population numbers.
[tag-tec]Live sand[/tag-tec] is a term which a lot of aquarists and non aquarists alike have quite often heard about. Live sand is sand which has been taken from the ocean and is therefore full of the valuable life which all aquarists crave for. Live sand can be used in a deep sand bed as long as the particle size is correct therefore ensure that you check this prior to adding it to your aquarium.
Ok that’s the grain size covered in what I have to say is in a little more detail than I had first envisaged. Lets go onto the next area which is the importance of depth.
Another shortcoming of under gravel filtration is that it does not have the ability to convert nitrates into nitrogen gas. This is because the water which is passing through the under gravel filter is actually moving relatively fast and is high in oxygen content, for this reason the anaerobic bacteria which are required to breakdown nitrate to nitrogen gas do not carry out this function as the conditions required are low oxygen and slow moving water.
When a sand bed starts to get deeper than 2+ inches then anaerobic areas will start to form. Anaerobic areas in a sand bed are areas which are very low in oxygen content, this is because the oxygen is being used by the aerobic bacteria above. Water flow through these lower areas occurs partly because of the burrowing activities of the organisms which live in the sand but is primarily due to an activity called diffusion. All fluids have a tendency to perform diffusion and is basically where two amounts of the same fluid which contain a different amount of chemicals balance themselves out over time via utilising the movement of molecules.
It is these anaerobic areas which allow for de-nitrifying bacteria to function and therefore convert nitrate into nitrogen gas which will escape the aquarium at the water surface.
So an effective deep sand bed has the ability, because of grain size, depth and various organisms, to permit both nitrifying and de-nitrifying bacteria to co-exist and continuously break down matter all the way through to the conversion of nitrate to nitrogen gas.
When a deep sand bed is combined with live rock in the marine aquarium any solid waste which settles upon the sand is broken down via the deep sand bed and the rest is cycled via the live rock’s excellent filtration capabilities – a match made in heaven.
Leaving live rock out of the equation though when a deep sand bed matures the following will happen:
- Waste is broken down when it lands upon the surface of the bed either via bacterial processing or via organism consumption.
- The upper layer is so oxygen rich that ammonia and nitrite is converted into nitrate.
- At the same time the nitrate concentration is being converted into nitrogen gas the nitrate created in the upper layers is ‘pulled’ down into the lower layers of the sand bed.
- When the nitrate reaches the lower levels it is converted into nitrogen gas.
- The nitrogen gas which is created rises through the sand and diffuses into the water column before being released into the air at the water surface.
- The cycle continues.
Therefore with a deep sand bed we have a never ending biological filtration cycle which is complete all the way through to the conversion of nitrate to nitrogen gas. The majority of other filtration methods stop after the creation of nitrate and other methods need to be introduced into the aquarium to control this remaining part of the cycle, whether this is water changes or the introduction of physical or chemical devices.
In a deep sand bed nitrate levels can be kept so low that even professional water test kits can struggle to achieve a reading (provided that other aquarium management techniques are adequate). This is very beneficial when the marine aquarist is attempting to maintain sensitive creatures.
Another benefit of a deep sand bed is the continuous release of planktonic larvae. These come from the reproductive activities of the organisms within the sand bed. Along with the very low nitrate levels which can be achieved there could also be a continuous supply of fresh food which is beneficial if you are keeping corals which require feeding. For this reason it is important that if the sand bed is not located in the display aquarium but in a sump, it is placed after the skimmer otherwise this fresh food could be skimmed out of the water.
Building a deep sand bed is actually quite a simple process but it does depend upon where you are with your aquarium, i.e. are you upgrading an existing system or implementing a new one.
If you are an aquarist who is hoping to upgrade an existing sand bed system then I would recommend that you do not add more than 1 inch of sand depth per month. The reason for this is that you need to give the organisms which already exist in your existing sand time to move upwards in the sand. If you add say 4 inches of sand in one big go then the organisms would not have had the chance to climb to safety before their oxygen ran out.
A better way in my opinion is to actually remove any existing sand from the system and start with a fresh sand bed.
Building a sand bed from scratch rather than upgrading really is a simple task to undertake. All you do is purchase the sand of the correct granular size, clean it and put it into your aquarium around the live rock up to the required depth. Once you have added the sand you will need to seed the sand bed with some organisms. This can be done by either purchasing some ‘reef grunge’, obtaining some sand from a fellow aquarists deep sand bed or getting some of the grunge from your local fish shops live rock curing vats.
With the introduction of the above and the eventual migration of creatures from the live rock into the sand all you have to do is let good old mother nature get to work.
If you are adding sand into an established tank then you will probably experience a small dust storm. This can easily be removed via mechanical filtration and any dust which has settled on either the corals or live rock can easily be blown off.
There is a very important thing that you must perform prior to putting any sand into your tank though and that is to give the new sand a very good soaking in old aquarium water. When I say a good soak I mean along the lines of 1 to 2 weeks. The reason for this is because of any chemicals which may have become bonded to the sand while it has been in the bag. Soaking the sand will remove these chemicals and stop them from entering the aquarium.
Now that you have a good idea as to how the deep sand bed came into fruition, the importance of granular size, how to build one etc lets quickly go over a few tips.
- Small size sand grains below 0.2mm and above 0.05mm must be used. Using sand of the correct size will allow the substrate to be turned over.
- Some heavier material (Small live rock no larger than 1cm) may need to be placed on top of the sand if you have a high flow aquarium otherwise the sand will literally blow away and you will have a water based sand storm to deal with.
- The depth should be greater than 3 inches to create the required anaerobic areas which are essential for the conversion of nitrate to nitrogen gas. The average depth in the majority of aquariums is between 4 and 6 inches as no benefit has been seen in sand beds greater in depth than this.
- Diversity and population will decide if a deep sand bed will filter effectively. The sand bed should be alive with various types of life (copepods, amphipods, brittle stars, bristle worms etc). There are literally hundreds of different species which dwell in and around sand beds serving similar and different roles. The higher the population of organisms the better the sand bed will be able to filter which makes us happy as aquarists.
- Increase diversity whenever possible – we do not wish one or two species to become dominant.
- Avoid fish and other animals which feed upon life in and on the sand bed as in no time at all the life forms could be decimated. A small amount of predation will occur and this of course is the natural way.
Passive sand sifters are a good idea as they help to keep the upper levels clear as well as promoting the re-growth of bacteria in these areas. I recommend that you keep the addition of these sand sifters low as they may remove the food source from getting to other organisms in the sand bed.
- Feed the organisms in the sand bed. Feeding increases both diversity and the population. A deep sand bed can deal with a large amount of food. When I say food I do not mean placing fish food etc on the sand bed I mean the food in the water, however feeding frozen fish food to the sand bed is good when you first start the bed as it increases the population. Do not just give the sand bed clean filtered, skimmed water ensure that it does get water which is dirty – i.e. un-filtered and un-skimmed.
There is a debate going on all over the marine aquarist world at present in relation to the possible longevity of the deep sand bed and the possible build up of metals. As of yet the build up of metals in a sand bed has not been proven and Dr Ron Shimeck is performing various experiments to ascertain answers. It has been suggested that it may be necessary to replace the sand bed every five years or so to remove this build up of metals, however again this is yet to be proven.
It is my firm belief that a deep sand bed combined with a live rock filtration system is one of the best natural filtration methods currently available to the marine aquarist. Remember to ensure that the particles are of the correct size, the depth of the bed is correct and that there are loads and loads of detrivores in the bed. If you don’t have enough detrivores then the bed will not be kept loose and cease to function.
Oh by the way don’t forget to purchase a torch if you do decide to implement a deep sand bed so that you can watch all the lovely beasties running all over the place at night time. It’s great fun!