In a fish-only marine tank, the water movement provided by your filtration system should be all you need for oxygenation. In a reef tank however, water movement takes on many other roles and needs to be considerably increased, and has to come from several sources.
What does water movement do?In a natural reef environment there is constant wave action creating water movement around the whole reef, and most corals have evolved to rely on, and take advantage, of this movement. The main two functions of water movement in a reef tank are to remove debris and carry food sources to the corals.
Debris removalMost reef aquariums have a significant amount of rockwork and even when using a reef-rack to allow an open space behind the visible rocks, there will be areas that will not receive a flow of water from the main filter pump. Left un-moved, these areas can become 'dead' spots, where oxygen levels drop and waste materials build up. Not only does this create toxic regions but it also prevents many of the useful organisms which can be found on live rock from staying alive and growing. In addition to dead spots forming, if general waste debris is allowed to settle on the soft tissues of many corals, the coral tissue and algae within the tissues can be damaged, and the corals health will deteriate.
Food sourcesMany corals will need additional food sources, which they will obtain by catching particles in the water. In order for corals to feed properly, there must be a sufficient quantity of these food particles suspended in the water, and it must be continually moved around so that it can be 'delivered' straight into the catchment area of the corals. Additional water movement is therefore vital to avoid dead spots, prevent settling on coral tissue, and to deliver food particles to the corals.
How to create water movementThe simple answer to creating the right water movement is to have as many pumps as possible, but we are limited by practicality constraints. In nano tanks it is possible to get away with as little as two pumps, including one from the filter, but for larger aquariums up to 90-120cm (3-4 foot) you should have a couple of large, main pumps, and at least two smaller pumps. The main pumps can be used for general circulation and should ideally be aimed at each other from opposite ends of the tank to create a highly agitated region in the middle. Secondary pumps can be positioned anywhere and can be used to either suck water away from, or push water into, areas which are likely to be dead spots. The pumps, or powerheads, should have a small grill or pre-filter sponge to prevent fish and inverts from getting sucked in, and if hidden amongst rockwork, should be somewhere easy to access for maintenance.