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03/18/2006, 04:30 PM
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Deep Sand Beds
One approach to tank filtration is the live deep sand bed, often called a "DSB". The goal of such a filter is to use organisms in the substrate to consume and export nutrients via various pathways. Nitrogen, for example, can be exported in gas form after processing by anaerobic microbes. Another advantage stated for live DSBs is that the animals living in the sand will produce eggs and larvae that will serve as food for other organisms in the tank. Stony corals, such as Acropora, are quite likely to feed on such items.
A depth of 6" (15cm) is usually the deepest that is suggested, with 4" (10cm) a likely minimum for reasonable effectiveness. The sand must be chosen carefully for particle size. Most or all commercial sand mixes are too coarse, so some careful research is suggested. This sample distribution (given in millimeters) is a good starting point:
Once the sandbed has been added to the tank, it must be stocked with animals. High-quality live sand and detritivore kits are good approaches. These animals typically must be restocked every 1-2 years since many organisms cannot form self-sustaining populations in an aquarium, although some aquarists consider this restocking unnecessary. In general, the substrate should cover the bottom of the display tank to form an effective filter, and the tank should be at least 30g (150 L) in size, and preferably larger. Smaller tanks cannot support the full array of animals required for a fully functional live sandbed. Deep sand beds require good flow. The flow only needs to be low enough to allow the sand to remain undisturbed. A good protein skimmer is usually recommended as well. Skimmers provide an export path for many of the wastes after processing by the sandbed fauna. Many animals are incompatible with a DSB. Sand-sifting starfish and hermit crabs will eat the animals that allow the bed to functions. Burrowing animals also can disturb the sand layer enough to prevent formation of anoxic regions important to denitrification, and often eat sandbed animals along the way. One issue with DSBs is that they place a lot of sand in the display tank. In many ways, having the filtration sandbed in a separate tank from the display would be desirable. This approach leaves more space in the display tank, and allows disconnecting the sandbed should any problems arise. Unfortunately, two issues limit the usefulness of this approach:     1) Moving fish waste and other detritus to a remote live DSB isn't feasible in many cases.     2) The area of the live DSB needs to be near that of the display tank in size to be effective. One problem cited with DSBs has been tank "crashes". In cases like this, the sandbed is said to fill with nutrients and then leach them into the water, or become covered with algal or microbial growth. This process occurs in nature and can lead to eutrophy in bodies of water. What the precise problem is and the actual causes are debated, but removal of the sandbed might be required. As with all filtration systems, keeping the feeding load within the processing capabilities of the aquarium is likely a key issue. Restocking of the sandbed infauna might also be an issue, along with incompatible stocking selections. Another approach, sometimes (rather confusingly) called a "remote DSB" or a "DSB in a bucket", works in a different manner. This method is beyond the scope of this article, since it is not really a substrate choice. When installing the tank, please be aware that any sand substrate can cause cloudiness when water is added. This condition is temporary and will clear on its own in a few days. A bit of live rock might help the process by seeding the tank with bacteria. The cloudiness can be reduced by covering the sand before pouring water into the tank, to reduce the disturbance. Reading this introduction is a good next step for setting up a DSB system. This book might also be useful.
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Jonathan Bertoni |
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