Circulation is one of many areas in the hobby where there is much discussion, which is understandable as circulation is important for the well being of the organisms in our care. In this article, I will discuss the importance of circulation, how much should be provided and the different ways it can be provided. As with most aspects of the hobby there are multiple methods to achieve the same end and one method is not necessarily more right than another. With a good understanding of the principles, the hobbyist can decide what is best for them and their aquarium.
Good circulation is necessary to ensure all areas of the aquarium 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 aquarium, 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 aquarium.
Water flow has been shown to be particularly important for photosynthesis, respiration and growth in corals (Dennison and Barnes, 1988; Jokiel, 1978), however it has been demonstrated that different species (Jokiel, 1978) and different colonies of the same species (Lesser et al., 1994; Montebon and Yap, 1995) can adapt to different water flow regimes.
Circulation needs to match the inhabitants of the aquarium. A aquarium with few inhabitants, and especially one with out much algae, may get by with less circulation. For example, a sea horse aquarium generally requires less circulation, but also water movement needs to be kept at a minimum because the sea horses are poor swimmers. For aquaria other than those set up for specific organisms such as sea horses, circulation needs to be at least vigorous.
As a general rule of thumb, the minimum circulation should be at least ten times the aquarium volume per hour. That is, for a 400 litre aquarium, the minimum should be around 4000 litres per hour in circulation. The circulation should be configured so that all areas of the aquarium receive water movement but with varying rates in different parts of the aquarium. This should ensure sufficient oxygen is distributed to the areas in the aquarium it is needed and carbon dioxide is taken to the water surface to be lost to the air around the aquarium.
While ten times aquarium volume is a good starting point, more circulation may be required depending on the inhabitants of the aquarium. A standard Fish Only With Live Rock (FOWLR) aquarium will usually do fine with 10X flow. Fish generally don't have any specific water movement requirements other than meeting their needs for respiration, which 10X flow can accommodate. Most invertebrates have no specific circulation requirements, either. Corals, on the other hand, can be more demanding.
Corallimorphs, zoanthids, most soft corals and some stony corals do not require much flow, although most of these will tolerate quite high flow if it is provided. Some stony corals such as Euphyllia, Catalaphyllia, Physogyra and Heliofungia may not fare well with high flow as their tissues can be damaged. Most other stony corals will do better with higher flow rates and in particular Acropora colonies from reef fronts and reef crests will do significantly better with up to 30 times aquarium volume per hour or even greater. It is important to thoroughly research potential inhabitants to ensure that their requirements can be met.
There are numerous ways to provide current in a aquarium. Each method has advantages and disadvantages and there is no perfect method to provide circulation.
Powerheads are probably the most straightforward method to provide water movement inside the aquarium.. Multiple powerheads can be placed in the aquarium 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 aquarium to reach the appropriate total flow. For example, 4 x 1000L powerheads would suit a 400L aquarium.
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 aquarium or a aquarium 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.
An alternative to powerheads is closed loop circulation . With closed loop circulation a larger water pump is used outside the aquarium, but the inlet and outlet lines are run between the aquarium and the pump. With the simplest form of closed loop circulation, only the piping is visible. With a little more planning, before the aquarium is setup, it is possible to drill the aquarium such that the piping for the closed loop circulation goes through the back, sides or bottom of the aquarium. 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.
Closed loop systems make it easier to hide the circulation devices, but the aquarium is still left with constant unidirectional flow.
One potential solution to laminar flow are products with rotating arms. One such product is the Sea Swirl Rotating Return Device. The Sea Swirl product works by slowly 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.
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.
One of the best devices for providing water flow that simulates natural water movement is a surge device. These are sometimes called dump buckets.
References
Dennison W.C. and Barnes D.J. 1988. Effect of water motion on coral photosynthesis and calcification. J. Exp. Mar. BioI. Ecol. 115:67-71.
Jokiel P.L. 1978. Effects of water motion on reef corals. J. Exp. Mar. Biol. Ecol. 35:87-97.
Lesser M.P., Weis V.M., Patterson M.R. and Jokiel P.L. 1994. Effects of morphology and water motion on carbon delivery and productivity in the reef coral, Pocillopora damicornis (Linnaeus): Diffusion barriers, inorganic carbon limitation, and biochemical plasticity. J. Exp. Mar. Biol. Ecol. 178:153-179.
Montebon A.R.F. and Yap H.T. 1995. Metabolic responses of the scleractinian coral Porites cylindrica Dana to water motion. I. Oxygen flux studies. J. Exp. Mar. Biol. Ecol. 186:33-52.
Last updated: January 5, 2005