As marine aquarists, most of us have at the very least heard of Deep Sand Beds (or DSB's for short). A lot of us have employed to great success. In roaming the marine aquarium online communities, anyone will quickly discover that a lot of the questions asked by newcomers to the hobby about DSB's are repeated very often. This article will attempt to answer such frequently asked questions and at the same hopefully give a brief introduction of how to set up a DSB for the beginner.
*** PLEASE NOTE: This article is based entirely on my view, opinion and experience. I won't attempt to prove every statement as each section/statement/fact can be researched by the reader separately as there are comprehensive texts about DSB on the World Wide Web. I am not a scientist by any stretch of the imagination, just a keen hobbyist.
WHAT IS IT AND WHY IS IT USED?
From its commonly used name, Deep Sand Bed, it is quite obvious that a DSB is simply a bed of sand that is deep. Deep enough so that the dissolved oxygen in the water that it is in, is very low. The primary requisite of anaerobic bacteria is satisfied when a low enough level of oxygen is reached. Certain anaerobic bacteria will start to thrive in this low oxygen environment. These bacteria will then start to consume Nitrate present in the water column. The product of the consumption of the Nitrate is Nitrogen which is released into the air in the form of bubbles. The consumed Nitrate is the end product of the aerobic cycle which any marine aquarist should already be aware of.
Nitrate, although far less toxic to fish and invertebrates, can become dangerous in high concentrations, hence the need for method to reduce the Nitrate concentration in the aquarium water. A DSB in the context of marine aquarium keeping can then be loosely defined as a container containing a deep bed of substrate of the correct particle size and that is connected to the aquarium in such a way to have a sufficient flow of water OVER the sand bed. But how deep is deep enough, how big must the particle size be how much flow over the DSB is enough? More on those questions in the following sections.
CONTAINER
The substrate must obviously be contained in some sort of container. The most typical application of a DSB is a compartment inside an existing sump. Another is to have the DSB in a separate tank or drum. In choosing what type of container to use, the decision comes mostly down to practicality and ease of installation. The reader should research over-tank sumps as it will influence the plumbing design beyond the scope of this article.
The generally recommended surface area of the DSB is considered to be about 3/4 the surface area of your main tank. I say generally recommended because there are diverse opinions about the minimum size of a DSB but no concrete scientific guidelines. As for the maximum size, bigger is better. The bigger a DSB is, the bigger its capacity to consume Nitrate and the lower the Nitrate concentration is. Also see the section about secondary benefits of the DSB.
FLOW RATE
As said before, the aquarium water should flow over the DSB and not through it. Pushing water through the sand bed will make it nothing more than an oxygen rich sand filter. No anaerobic bacteria would populate the sand bed and consequently no Nitrate reduction would be achieved.
Flow rate will be largely influenced by the size of your DSB container but also what you want to keep on top of and in your DSB. Again it is difficult to define an ideal value. Most sources indicate the aquarist should aim to achieve a steady, non turbulent flow over the DSB of between 10x and 20x the total system volume. You can keep an eye out for signs of too little flow. One such sign is the formation of Cyanobacteria on the DSB substrate.
SUBSTRATE
This is probably the single most important factor to consider when setting up a DSB. There are basically three properties to consider when choosing the right substrate.
Chemical Makeup - Any substrate suitable to be used in a marine aquarium should be good to use in a DSB as far as chemical makeup goes. The substrate must not contain any impurities like heavy metals that can leach back into your aquarium water.
Particle Size - As particle size of a substrate decreases, so its total surface area increases. An increased surface area means a bigger space for bacteria to grow on. The more bacteria you can get to grow the more Nitrates can be consumed. The generally accepted recommendation for particle size is to use a sugar fine substrate. Another important aspect is the use of a substrate with a more or less even particle size.
Particle Shape - Ideally the shape of particle to use is round. Substrate that consists of small round particles (generally called an oolitic substance) is ideal. Round particles prevent clumping of the substrate which can cause dangerous sulphide to form.
Crushed Coral - Not recommended for use as certain texts claim that the sharp edges of crushed coral can cut and damage some of the life forms that would typically be found on and in a DSB.
Sugar Fine Aragonite - Probably the most ideal substrate to use for DSB's but it is very expensive. It is claimed that aragonite acts as a pH buffering addition to the DSB benefits. Whether or not aragonite adds sufficient buffering capability to the aquarium to justify its price is beyond the scope of this article.
Pool Filter Sand - Another good option for use in a DSB as long as the particle size is near sugar fine. Most pool filter sands are silica based. Some sources claim that this can leach silica into your water column and cause algae blooms but I am not aware of any scientific evidence supporting this theory.
Play sand - Also a silica based sand. This is one of the cheapest substrates to use and is typically of the correct particle size and shape. An additional benefit to using this type of substrate is that the sand is usually prewashed and guaranteed not to contain contaminants or any cancer causing elements (due to the health risk to the children that will be playing in it).
Beach Sand - Beach sand is great as long as the correct particle size is used. Care should be taken to rinse the beach sand thoroughly before adding to the DSB container.
Very often aquarists use a combination of different substrates
LIFE IN THE DSB
A DSB cannot work if only consists of substrate. We need animals to constantly disturb the top layer of the sand bed. This is achieved by various types of digging worms, small crustaceans, tiny brittle stars and small crabs and snails. Although some critters will disturb the substrate keeping it churned, it is also important to have a good concentration of scavengers. Scavengers will consume detritus before it can start to rot. Try to avoid aggressive predators in the DSB as it will effectively wipe out your other beneficial life forms.
MACRO ALGAE AND LIGHTING CYCLE
A good practice is to add a macro alga like Chaetomorpha or Caulerpa to your DSB. This serves the benefit of extra nutrient export from your system as well as a nice home for many pods and worms. Macro algae can be housed almost anywhere where vegetarian fish can't reach and eat it. But having a DSB simply makes for a convenient place to have your macro algae.
A strategy employed by many hobbyists, is to have a reversed light cycle over your DSB. In other words, if the main display's lights are on, the sump/refugium/DSB's lights are off and vice versa. The effect of doing this is a more stable pH. Normally, in a reef tank, the pH of the water will drop slightly over night as the photosynthetic organisms (most coral, algae etc.) will produce CO2 that is released into the water. The rise in the CO2 level will cause a drop in pH. To counter this effect, the photosynthetic macro algae in the sump, will take up this CO2 released by the corals and algae of the display, causing a more stable pH.
SECONDARY BENEFITS
Apart from the benefit of achieving Nitrate reduction with a DSB, there are a couple secondary benefits worth mentioning.
Food Source - A well populated DSB will continuously produce pods and other crustaceans that will get consumed by your fish as they get transported by return pumps. Most of the fauna in a typical DSB will also produce phytoplankton in the form of larvae. This is a very good source of food for corals.
Water Volume Increase - Adding a DSB will increase the total water volume of your setup making parameters generally more stable.
*** PLEASE NOTE: This article is based entirely on my view, opinion and experience. I won't attempt to prove every statement as each section/statement/fact can be researched by the reader separately as there are comprehensive texts about DSB on the World Wide Web. I am not a scientist by any stretch of the imagination, just a keen hobbyist.
WHAT IS IT AND WHY IS IT USED?
From its commonly used name, Deep Sand Bed, it is quite obvious that a DSB is simply a bed of sand that is deep. Deep enough so that the dissolved oxygen in the water that it is in, is very low. The primary requisite of anaerobic bacteria is satisfied when a low enough level of oxygen is reached. Certain anaerobic bacteria will start to thrive in this low oxygen environment. These bacteria will then start to consume Nitrate present in the water column. The product of the consumption of the Nitrate is Nitrogen which is released into the air in the form of bubbles. The consumed Nitrate is the end product of the aerobic cycle which any marine aquarist should already be aware of.
Nitrate, although far less toxic to fish and invertebrates, can become dangerous in high concentrations, hence the need for method to reduce the Nitrate concentration in the aquarium water. A DSB in the context of marine aquarium keeping can then be loosely defined as a container containing a deep bed of substrate of the correct particle size and that is connected to the aquarium in such a way to have a sufficient flow of water OVER the sand bed. But how deep is deep enough, how big must the particle size be how much flow over the DSB is enough? More on those questions in the following sections.
CONTAINER
The substrate must obviously be contained in some sort of container. The most typical application of a DSB is a compartment inside an existing sump. Another is to have the DSB in a separate tank or drum. In choosing what type of container to use, the decision comes mostly down to practicality and ease of installation. The reader should research over-tank sumps as it will influence the plumbing design beyond the scope of this article.
The generally recommended surface area of the DSB is considered to be about 3/4 the surface area of your main tank. I say generally recommended because there are diverse opinions about the minimum size of a DSB but no concrete scientific guidelines. As for the maximum size, bigger is better. The bigger a DSB is, the bigger its capacity to consume Nitrate and the lower the Nitrate concentration is. Also see the section about secondary benefits of the DSB.
FLOW RATE
As said before, the aquarium water should flow over the DSB and not through it. Pushing water through the sand bed will make it nothing more than an oxygen rich sand filter. No anaerobic bacteria would populate the sand bed and consequently no Nitrate reduction would be achieved.
Flow rate will be largely influenced by the size of your DSB container but also what you want to keep on top of and in your DSB. Again it is difficult to define an ideal value. Most sources indicate the aquarist should aim to achieve a steady, non turbulent flow over the DSB of between 10x and 20x the total system volume. You can keep an eye out for signs of too little flow. One such sign is the formation of Cyanobacteria on the DSB substrate.
SUBSTRATE
This is probably the single most important factor to consider when setting up a DSB. There are basically three properties to consider when choosing the right substrate.
Chemical Makeup - Any substrate suitable to be used in a marine aquarium should be good to use in a DSB as far as chemical makeup goes. The substrate must not contain any impurities like heavy metals that can leach back into your aquarium water.
Particle Size - As particle size of a substrate decreases, so its total surface area increases. An increased surface area means a bigger space for bacteria to grow on. The more bacteria you can get to grow the more Nitrates can be consumed. The generally accepted recommendation for particle size is to use a sugar fine substrate. Another important aspect is the use of a substrate with a more or less even particle size.
Particle Shape - Ideally the shape of particle to use is round. Substrate that consists of small round particles (generally called an oolitic substance) is ideal. Round particles prevent clumping of the substrate which can cause dangerous sulphide to form.
Crushed Coral - Not recommended for use as certain texts claim that the sharp edges of crushed coral can cut and damage some of the life forms that would typically be found on and in a DSB.
Sugar Fine Aragonite - Probably the most ideal substrate to use for DSB's but it is very expensive. It is claimed that aragonite acts as a pH buffering addition to the DSB benefits. Whether or not aragonite adds sufficient buffering capability to the aquarium to justify its price is beyond the scope of this article.
Pool Filter Sand - Another good option for use in a DSB as long as the particle size is near sugar fine. Most pool filter sands are silica based. Some sources claim that this can leach silica into your water column and cause algae blooms but I am not aware of any scientific evidence supporting this theory.
Play sand - Also a silica based sand. This is one of the cheapest substrates to use and is typically of the correct particle size and shape. An additional benefit to using this type of substrate is that the sand is usually prewashed and guaranteed not to contain contaminants or any cancer causing elements (due to the health risk to the children that will be playing in it).
Beach Sand - Beach sand is great as long as the correct particle size is used. Care should be taken to rinse the beach sand thoroughly before adding to the DSB container.
Very often aquarists use a combination of different substrates
LIFE IN THE DSB
A DSB cannot work if only consists of substrate. We need animals to constantly disturb the top layer of the sand bed. This is achieved by various types of digging worms, small crustaceans, tiny brittle stars and small crabs and snails. Although some critters will disturb the substrate keeping it churned, it is also important to have a good concentration of scavengers. Scavengers will consume detritus before it can start to rot. Try to avoid aggressive predators in the DSB as it will effectively wipe out your other beneficial life forms.
MACRO ALGAE AND LIGHTING CYCLE
A good practice is to add a macro alga like Chaetomorpha or Caulerpa to your DSB. This serves the benefit of extra nutrient export from your system as well as a nice home for many pods and worms. Macro algae can be housed almost anywhere where vegetarian fish can't reach and eat it. But having a DSB simply makes for a convenient place to have your macro algae.
A strategy employed by many hobbyists, is to have a reversed light cycle over your DSB. In other words, if the main display's lights are on, the sump/refugium/DSB's lights are off and vice versa. The effect of doing this is a more stable pH. Normally, in a reef tank, the pH of the water will drop slightly over night as the photosynthetic organisms (most coral, algae etc.) will produce CO2 that is released into the water. The rise in the CO2 level will cause a drop in pH. To counter this effect, the photosynthetic macro algae in the sump, will take up this CO2 released by the corals and algae of the display, causing a more stable pH.
SECONDARY BENEFITS
Apart from the benefit of achieving Nitrate reduction with a DSB, there are a couple secondary benefits worth mentioning.
Food Source - A well populated DSB will continuously produce pods and other crustaceans that will get consumed by your fish as they get transported by return pumps. Most of the fauna in a typical DSB will also produce phytoplankton in the form of larvae. This is a very good source of food for corals.
Water Volume Increase - Adding a DSB will increase the total water volume of your setup making parameters generally more stable.
