The ocean is a turbulent environment, water is constantly being mixed by oceanic currents, wind and the waves that result. Such phenomena are essential to the world's oceanic ecosystems. Without water circulation, the world's oceans would soon stagnate. Powerful ocean currents help to transport nutrients and delicate microorganisms such as phytoplankton to different parts of the globe. The Gulf Stream (which spans from the waters off the Florida coast and north-east towards Northern Europe) moves 30 million cubic metres of water every second.

One ocean habitat is particularly reliant on water circulation: coral reefs.

Water movement is also essential in the home reef aquarium. All of the living organisms within the aquarium rely on oxygen to survive, just as we do. The movement of water transports water that has been depleted of oxygen towards the water surface where surface agitation helps oxygen from the atmosphere to dissolve into the water column. Most simple aquatic microorganisms and many invertebrates have passive breathing systems which rely on strong water movement to bring oxygenated water and remove carbon dioxide. Many species of fish and nearly all sessile invertebrates including corals, anemones, sponges and feather dusters rely on water circulation to bring a steady supply of food.

The movement of water also helps to maintain the chemical equilibrium within the aquarium and enchances biological filtration by supplying important bacteria with oxygen. Carbon dioxide which is produced by respiring aquarium organims is mildly acidic and if it does manage to accumulate to high levels, it may cause the aquarium's pH to drop and cause the fish to develop a condition known as blood acidosis as a result.

Different coral species have differing requirements where water circulation is concerned, just as they have different requirements for lighting and placement within the aquarium. For example, many species of SPS (Small Polyp Stony) corals require strong, random water movement to ensure they receive enough oxygen, essential calcium for skeletal growth and that the corals are kept clear of sediment deposits so that the Zooxanthellae in their tissues can photosynthesize effectively.

SPS corals can thrive in tank turnover rates ranging from as little as 15 times per hour to over 130 times per hour. The intensity of water circulation has a profound and direct impact on the manner in which SPS corals grow. In high water movement, Acropora corals cluster themselves into more compact colonies with shorter and stubbier branches. The branches are typically stronger as the corals have to absorb more calcium to increase the strength of their skeletons so they don't break in turbulent conditions.

Soft corals and LPS (Large Polyp Stony) corals on the other hand don't normally require such high turnover rates, but the requirements do vary from species to species.

Below I will discuss some of the ways in which adequate water movement can be implemented.

Circulation pumps - The typical choice for the bulk of reef-keepers. Pumps have become more powerful and more energy efficient in recent years, as demonstrated by models from Tunze and the Vortech pumps from EcoTech Marine.

As well as having pumps inside the actual aquarium, using circulation pumps also includes the sump pump if your reef has a sump tank where equipment such as protein skimmers, calcium reactors and the usual gadgetry are kept. It is important to consider the loss of water flow as the pumping head distance between the sump and aquarium increases.

Closed-loop system - Many fishkeepers are left bewildered as to what a closed-loop really is. Basically, it's a 'dry' external pump which sucks water from the aquarium and circulates it back to the aquarium through solid PVC piping with numerous outlets around the tank.

Wave-makers or wave boxes - The most well-known wave-making device is the Tunze wavebox. This is now available in a nano-sized version. The constant sloshing of water from one end of the tank to the other accurately stimulates the same movement of water generated by waves on a coral reef. They are expensive, but for the aesthetic appeal and the benefits for SPS keepers, wave-boxes are worth considering.

Finally, don't just consider the devices that generate water movement. Think about the placement of rocks in the tank. Poorly-placed rocks create dead-spots of stagnant water where most corals and sessile invertebrates simply won't thrive. An open rock structure will ensure that enough water movement reaches all corners of the reef.

Wow you have a great knowlege or marine fish. I have been researching about marine fish as I would like a tank but I have read that anenomes are very hard to keep as they require very clean water and the right lighing. I wanted an anenome for my clownies can you recommend a beginners anenome that is a little hardier?
Cheers

IMO you would be better off with one of the Euphyllia corals than an anenome. They are much easier to keep, don't wander around the tank and clowns will live in them
http://www.youtube.com/watch?v=-e3rEwBlNdo

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As Hawksport has pointed out, don't get an anenome - they're notoriously difficult to keep.

To be honest, most Clownfish are captive-bred and would never have even seen an anemone. Some might form a symbiotic relationship at first sight of an anemone or other suitable host (such as a Euphyllia coral), but most will probably ignore it.