One thing that fish and corals are very used to is seawater movement. The fish are well adapted to it and corals also use it for the capture of food and the disposal of waste. It follows that there should be reasonable circulation in the marine aquarium, particularly so with a reef aquarium. Seawater movement is also important for oxygen. Fresh water holds less oxygen than air, heated fresh water less and seawater even less. Oxygen is taken in at the seawater surface, so adequate movement ensures that seawater at the surface is changed continually maintaining adequate oxygen content.
There are three types of movement, turbulence, laminar flow and surge. The first, turbulence, is the random movement of seawater in all general directions. The second, laminar, is flow in one particular direction. The final type, surge, is where the movement is forward and then back again repeatedly.
Considering the least used first, surge, this is created by using special pumps that are designed to switch on and off repeatedly, thus the seawater is forced forward and then naturally flows back. On the surface of the aquarium this can cause a wave effect. Pumps could have built in direction change up to a point, so on each on period the seawater is pushed in a different direction. The direction ability is limited of course. The movement is fairly natural but more strictly rhythmic than nature’s own. These pumps are not used by many aquarists.
Turbulence is a useful type of movement and is created within the aquarium reasonably easily along with laminar flow. The most common pump, the power head, is used. Basic power heads are submersible electric pumps that constantly pump seawater in one direction. They are sold in different strengths being capable of pumping different amounts of seawater, often stated in litres per hour. So how is the strength of the power heads determined? The general amount for movement in the aquarium is 10 times per hour of the net gallonage excluding any sump (more for hard corals). So a net 40 gallon aquarium would be around 400 gallons per hour, the 400 being divided by the number of pumps, eg. 4 pumps would be around 100 gallons per hour each. This is not a strict rule but a guideline. As said, pumps often have their output stated in litres per hour so the answer is easy (though not exact, one gallon = 4.5 litres). Excessive pumping power needs to be avoided. Wait though, these pumps produce laminar flow, not best for the aquarium. Power heads are the most popular with aquarists so lets have a look at placement to produce laminar flow and also turbulence.
Power heads are easily placed anywhere in the aquarium, high, low, in corners on flat surfaces. The output direction can be adjusted. In a smaller aquarium two would suffice, one in each rear corner. As said, the flow leaving the power head is laminar and corals won’t appreciate this, so the output flow of both pumps is broken by pointing the pump at a reasonably close flat glass surface so the flow output hits the surface at an angle, the front glass is often used. The flow bounces off the glass and changes into a wider more multi-directional movement. The flow from the other pump does the same being altered in the same way on another flat surface. The two flows interact with each other and create a multi-flow, or turbulence. If the pumps are of adequate power, the seawater movement throughout the aquarium is effected. There will still be a few areas of low movement but this is unavoidable. The important thing is the flow is suitable for the corals and comfortable for the fish. Once reasonable positioning of the power heads has been achieved then placement of corals is easier, making sure the corals have sufficient light and seawater movement that is adequate but not excessive.
What of the bigger aquarium? It’s the same thing but because of the greater distances involved the power heads would need to be more powerful. However, it isn’t that straight forward! If the power is too high then seawater movement could be excessive even if the pumps outputs were directed as in the previous paragraph. So an alternative is to use more normally powered pumps but have additional ones located in the middle of the aquarium. These could have their outputs directed forwards to the front glass or whatever is required to achieve turbulence. Again, once this is achieved the corals are placed ensuring they have suitable flow and light.
Placing the power heads does not have to be as described, it is a matter of practicality and then checking. The aquarium could be temporarily filled with tap water and the flow checked, remembering that when any rocks are placed the flow could alter. Whatever the size of aquarium, once the power heads are delivering adequate seawater movement the livestock can begin to be placed (assuming the aquarium has been properly initially matured). In the early weeks a check needs to be kept on the seawater movement making sure that corals are not being over exposed. Generally fish should be comfortable.
An example has been described using the cheapest option, power heads which are readily available. Once the power heads are correctly positioned the fish and any corals should be happy and settle. Given time, a lovely natural looking aquarium should evolve.