Incorporated into every reef tank should be two basic types of water circulation; Filter circulation and Internal circulation. Filter circulation is water that moves from the tank to the filter system and back to the tank. Internal circulation is the water that moves within the tank itself. There are many methods and theories for accomplishing both. Personally I believe lack of good Internal Circulation is one of the main reasons for algae problems.

Think about it, most reef tanks have an overflow system at the surface of the tank, if debris (eg; food, fecal matter and mulm) are not swept up from the bottom of the tank, or from behind the rocks by currents, then they never exit the tank, via the overflow, and are not removed by the filter system.

At its best, Internal circulation should consist of Surge ( back and forth movement), Turbulence (random swirling movement) and Laminar flow (movement in one direction) [ see Delbeek and Sprung, The Reef Aquarium, Vol 1, page 164 ]. Available are an array of devices to accomplish some of these types of movement.

Wave makers, usually an electronic switching device that turns on and off alternating pumps / powerheads, will generate Laminar flow and some Turbulence. Unfortunately pumps / powerheads were not intended to be turned on and off, and generally the shortest time duration / cycle is two minutes.

Oscillators, there are a few manufactures who produce units that either move your powerhead back and forth 90 degree's, or units with motorized water discharges that move back and forth 90 degree's. These do create "chaotic" Turbulence.

Dump Buckets, an offset bucket that fills and then tips to dump its water, then rights itself to refill. These units create Surge and Turbulence. I don't believe there are any commercial units marketed yet (review either Walter Adeys Dynamic Aquaria or there was an article in MFM a few years back ( Stanley Brown ) on how to make a Dump Bucket out of a paint roller pan.

THE ABOVE MENTIONED UNITS ALL HAVE MOVING PARTS, AND MURPHY'S LAW SAYS... < G >

Carlson Surge Device, (see SeaScope, Summer 1996 or CLICK HERE ), is an elevated tank that when filled will allow a rapid siphon to be created out of the tank and introduced into the reef tank. Once the elevated tank drains, the siphon is broken and the tank refills to create another rapid siphon. This will create Surge and Turbulence.

THE LATER TWO UNITS WILL CREATE MORE SPLASHING OF WATER AND REQUIRE A TANK / OVERFLOW / SUMP THAT CAN HANDLE A FLUCTUATION IN ITS WATER LEVEL.

All the units mentioned above will help suspend debris and particles into the main water column, which eventually will be carried out the overflow and into the filter system. With less debris remaining in the tank, there will become a much lesser chance that undesirable algae's will get a foot hold. Additionally, the corals in the tank will also benefit from the greater water movement, as they will be able to expel waste products and more food will be brought to them. Keep in mind that not all corals will appreciate the greater water flow, these corals can be moved into quieter areas of the aquarium. But after some adjustment, most corals and polyps will expand further, or open larger.
 

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