One of the major differences between Marine systems and freshwater systems is the amount of water flow required for the animals to thrive. Most freshwater animals come from areas of relatively calm water and even the ones that come from faster moving rivers need very little in the way of water movement provided proper oxygenation can be provided. Saltwater tanks on the other hand require a lot more water movement then their freshwater counterparts.
One of the most obvious and important reasons for having good water movement in a tank is for gas exchange. Due to the nature of saltwater and the temperature we keep them at it is hard to keep the oxygen levels high for the animals. A build up of CO2 in the system has an extra disadvantage of driving the pH down in the tank and eventually exhausting the alkalinity.
With the large amount of rock generally found in a SW tank, you are faced the problem of debris getting caught up in all the small places in and around the rocks. To prevent this from happening it's important to provide a lot of random water movement in the tank. Ideally, this random water movement will carry the debris away from the rocks and towards an area in the tank where it can be easily removed through vacuuming or using a mechanical filter before the water is further processed. Conventional methods of providing water circulation such as powerheads, have a major flaw. Although they provide good water movement, it is generally unidirectional in nature, which will push all the debris in one direction and eventually you will get a build up on one side of the rocks which will be difficult to remove, thereby creating a possible nitrate source for the tank making water quality harder to maintain.
A lot of rockpool fish are found in areas were there is a lot of turbulent water flow coming at them from all directions.
One of the biggest misunderstandings of water movement in the tank is the thought that the turnover rate is the amount of water that needs to be removed from the tank, filtered and then returned to the tank again. In actuality the turnover rate refers to the amount of water that needs to be moved around in the tank including the movement from filtration units, skimmers, and pumps etc. The minimum turnover rate for a reef tank is 10 times the volume of the water in the tank with many tanks having much more water movement then that. It is vitally important to have no dead spots in the tank and the more turbulent the water movement the better it will be for keeping the animals healthy.
No matter how you chose to set up the water flow in your tank there are a couple things to keep in mind. The more turbulent you can make the water the better for your animals. Try not to rely on only one source of water movement in the tank for if that fails your animals will suffer greatly and by the time it's noticed it may be too late for the tank.
These are the most widely available method for introducing water movement within the tank. Powerheads are submersible pumps that pull water through them and use the tank water to help keep them cool. The use of powerheads will provide and extra heat source to your tank and care should be taken to ensure that this doesn't contribute to overheating your tank. Powerheads are great for moving large quantities of water however have a major disadvantage of only providing unidirectional water movement.
Pumps are external devices that need to be plumbed into the tank. They are more powerful then submersible powerheads so they can pump more water throughout the system. They still need to monitored as a heat source since most of the pumps available will use the water passing through it to help cool the motor.
Powerheads and the drains for pumps need to have protection installed so that animals and debris wont be sucked into the motor section causing the death of an animal or clogging of the propeller. They should also be directed so that the returning stream of water isn't directly blowing on animals as this narrow powerful stream could cause damage to animals. Ideally the stream should be dispersed as quickly as possible so animals wont be damaged.
These are generally electronic devices that are programmed to turn a series of powerheads on and off in a relatively random order to provide a more variable form of water movement. These can also be simulated with using a series of timers on each powerhead. Although these devices are an improvement over just having powerheads in the tank, they do have a few drawbacks. The first is that the time between turning the powerhead on and off is fairly long thereby just giving temporary relief from a unidirectional current. The second is that most powerheads aren't designed for being constantly turned on and off so this will significantly shorten the lifespan of the powerheads in use.
These are relatively new products on the market. It's a "T" shaped device that directs water from the left to the right using the water pressure from a pump to switch the direction of the water current. It is a none electrical device that can be used either submersed in the tank with a powerhead plumbed externally for use with a pump. The speed these devices switch direction is determined by the pressure from the pump. The stronger the pump the faster the device switches direction creating a more natural turbulence in the tank. They are relatively inexpensive and have a good reputation for being a reliable way to create turbulent water flow in a tank.
These are devices that utilize a powerhead or the return from the filters to help direct the water flow in the tank. A basic spray bar is a piece of PVC piping with many small holes drilled into it so that the force of the water stream is dispelled throughout a greater area. The holes should be drilled randomly around the pipe so that you get a maximum of different direction of water dispersal.
The spray bar can be located near the top of the tank to facilitate better gas exchange in the main tank, behind the rockwork to blow debris out from behind the rocks, or even at the bottom of the tank to help keep the substrate clear of debris. It's possible to use a combination of spray bar locations to ensure that there aren't any dead spots in the tank and to create currents that will cross paths providing additional turbulence.
This is an expansion of the spray bar system. Instead of just using the return flow or a submersible powerhead, it uses and external pump that removes water from the tank and then returns it without any additional filtration or minimal filtration or larger particles to prevent the pump from clogging. This enables stronger more powerful pumps to be used, eliminating the need for powerheads in the tank. This also enables devices such as the SQWD to be plumbed into the loop and creating a more random water flow then previously available. The closed loop system can be hidden behind the tank so intrusion of equipment will be minimal.
Surge devices are probably one the most effective methods of creating turbulent water flow within the tank, but are also one of the most difficult to set up and can be very intrusive. These devices are basically a container stored above the tank where water is constantly being pumped into the storage container. Once the container is filled to a certain point, the water is then released back into the main tank creating a very natural wave action. A couple major drawbacks of a surge device is that it is bulky and sitting it above your tank may be difficult if space is limited and wont look very pretty. They are also noisy when they release the water into the tank and finally they create a lot of air bubbles. Although the air bubbles wont damage most animals in the tank they can be unsightly to most people