Seawater movement in a marine aquarium, whether it is a fish only or reef system, is important for the overall health of the environment.
Adequate movement allows for oxygen intake at air/water interfaces, particularly the surface of the display aquarium. A high level of oxygen in the seawater is obviously good for fish and also for corals. The movement also helps corals shed mucus and brings food to them.
Is it possible to have too much movement? It is.
Generally, fish are well adapted to strong movement; their real home is the wild reef. Dependant on their position on the reef they are able to deal with all sorts of movement from chaotic to wave even to linear all at various strengths. The general guideline for seawater movement in a fish only system is 10 times the net gallonage of the display aquarium (the sump, if any, is ignored). This is easily achieved by sizing powerheads etc correctly.
A reef aquarium could be soft or hard (SPS) corals or a mixture. The general guideline for movement in a soft coral aquarium is the same as a fish only one. The requirement for a hard (SPS) coral reef is considerably higher, here the general guideline is 20 + times the net gallonage of the display aquarium. Again, this can be easily achieved.
Seawater movement in a mixed hard/soft coral reef usually caters for the SPS corals. Any soft corals are carefully chosen and placed so that they receive less movement. Low down on the reef is usually the place for them.
If the system includes a sump what seawater movement is required in that? In this case the general guideline is about 3 times the total system net gallonage per hour. The flow usually relies on a gravity feed at the ‘in’ end and a return pump at the other. This creates a straight through or linear flow. In a sump this doesn’t matter.
As is often said the flow type in the display aquarium should be random and chaotic. This is achieved by, for example, playing with the positions of the powerheads.
So what if the aquarist has made a mistake and has oversized the return pump in the sump? In this case livestock, apart from maybe algae, are unlikely to be affected. However, there is the possibility that if a deep sand bed (DSB) is in the sump the very fine sand is going to be taken from one end of the sump to the other, though many aquarists use baffles to prevent this. It would take considerable over sizing to create a real problem.
In a soft coral reef care is needed with water movement. If the movement is too strong then some of the corals will bend over like trees in a gale or worse. Fine sand from a DSB will blow about and land on rock decorations and the interference with the DSB will reduce or spoil its filtration action. Even decorative sand, which is coarse, could blow about.
With the SPS reef the corals demand more seawater movement and the likelihood of an excess is lower. As said, the guideline is 20+. In this type of system close attention needs to be paid to the type of sand used for the display, a DSB would be better off in the sump and decorative sand is likely to shift about. Close attention also needs to be given to the position of any soft corals.
What of the fish? With an SPS reef where the aquarist has provided very high seawater movement it is necessary to have fish which are able to deal with that movement. Some fish would not be suitable. Having said that, there are always fish that dwell around the rocks, and anyway the seawater current strength is not aquarium wide, such as parts close to and within the reef.
A very severely ‘over pumped’ display aquarium generates the vision of fish going round in circles as though in a washing machine, all looking most puzzled, and soft corals lifted from their anchorage and mashed by impellors! That’s not going to happen.
It is possible to provide excessive circulation. Aquarists generally consider livestock when thinking of circulation and this is of course quite correct. In addition though consider other aspects of the display such as sand. These decisions should preferably be made at the design stage.