Why is circulation important?

Good circulation is necessary to ensure all areas of the tank have a good supply of oxygen and low levels of carbon dioxide. Additionally, circulation is necessary for sessile organisms to bring them their food and to take their waste away.

Circulation should not be confused with aeration. Circulation moves water around the tank, taking it to and from an air/water interface. Aeration provides a site for gas exchange but may not provide much circulation. For example, a protein skimmer provides good aeration and the water that exits the skimmer will be saturated with oxygen and have little or no excess carbon dioxide. However, circulation is still required to take that oxygenated water to all areas in the tank.

How much circulation should be provided?

Circulation needs to match the inhabitants of the tank. A tank with few inhabitants, and especially one with out much algae, can get by with less circulation. For example, a sea horse tank generally requires less circulation, but also water movement needs to be kept at a minimum because the sea horses are poor swimmers. For most other tanks, circulation should be more rigorous.

As a general rule of thumb, you should aim to provide at least ten times the tank volume in circulation per hour. i.e. for a 400L tank aim for 4000L/hr in circulation. The circulation should be configured so that all areas of the tank receive water movement but with varying rates in different parts of the tank. Some organisms, e.g. stony corals from reef fronts and crests, may require even greater flow rates. Corals that come from areas of high flow generally have short thick branches.

How can circulation be provided?

There are numerous ways to provide current in a tank. Each method has advantages and disadvantages and there is no perfect method to provide circulation.

Powerheads

Powerheads are probably the most straightforward method to provide water movement inside the tank.. Multiple powerheads can be placed in the tank in different locations pointing in different directions. This is the easiest way to ensure good flow and is also the most flexible. To achieve the desired circulation, multiple powerheads are added to the tank to reach the appropriate total flow. For example, 4 x 1000L powerheads would suit a 400L tank.

Attaching powerheads can be a little troublesome. The suction cups provided with most brands of powerheads often fail after a relatively short period of time - as little as a month for some. The suction cups supplied with Maxi-Jet powerheads seem to last much longer. However, even the best ones eventually fail. It is quite easy to make holders for powerheads from clear acrylic (perspex).

Brands of powerheads available include:

The Maxi-Jet powerheads have a number of advantages over the other brands available. The supplied bracket can be attached to the powerhead on three out of four sides which gives more flexibilitt in mounting. The outlet can be rotated through 360° which gives far more control over the direction of flow. As mentioned above, their suction cups are more reliable.

Powerheads have a few disadvantages:

It is possible to hide the powerheads within the live rock, but the cords will still be visible. Hiding the powerheads also makes them difficult to clean.

The constant unidirectional flow from a powerhead (or multiple powerheads) provides suffcient circulation to keep oxygen levels up and carbon dioxide levels down. This is good in a fish only tank or a tank where the majority of the organism can move about. However, sessile organisms, such as corals, will be hit by a constant flow of water from one direction. This does not emulate the natural flows these organims would experience on a reef.

There are a number of devices that seek to address some or all of the disadvantages above.

Closed loop circulation

An alternative to powerheads is closed loop circulation . With closed loop circulation a larger water pump is used outside the tank, but the inlet and outlet lines are run between the tank and the pump. With the simplest form of closed loop circulation, only the piping is visible. With a little more planning, before the tank is setup, it is possible to drill the tank such that the piping for the closed loop circulation goes through the back, sides or bottom of the tank. This allows for the piping and the outlets to be hidden within the rock framework. For more information on closed loop circulation, see DBW's Water Circulation System.

Rotating arm devices

There are a number of products on the market that seek to address the laminar flow problem. One such product is the Sea Swirl Rotating Return Device and there are similar products available from other vendors. The Sea Swirl product works by rotating the water outlet through 180° such that the flow is continually changing. While this idea has merit, it doesn't address the real problem, that of corals, etc., receiving water flow in only one direction. As the outlet rotates past a coral, the coral will still receive flow only from the direction of the outlet. One way around this problem would be to install multiple rotation devices to generate flow from multiple directions, however, ultimately the flow will not be very natural.

Switching devices

Another potential solution to laminar flow are products called "wavemakers". "Wavemakers" by alternately switching powerheads on and off at different intervals to reduce the constancy of the flow. There are a number of brands available and they can also be built as a project, if you are electronically minded.

Surge Devices

One of the best devices for providing water flow that simulates natural water movement is a surge device. These are sometimes called dump buckets.


Last updated: October 16, 2002