Seawater movement in the aquarium is a known requirement. This movement is needed by all marine aquarium creatures to a greater or lesser extent.
The aquarist who keeps a fish only system may believe that seawater movement is not so important and that belief is correct. However, fish need reasonable movement to prevent an area of ‘dead’ seawater developing around their body. It is also much more natural to have healthy movement as this is after all what occurs in their natural habitat, the wild reef. Fish also have a strong demand for oxygen and seawater movement maximizes good gas exchange.
Corals have a greater demand for seawater movement generally. This demand does vary between types, for example soft corals, again generally speaking, need less flow than hard corals, particularly the SPS (small polyp stony) types. As a general guideline to seawater movement, in the aquarium a soft coral display requires around 10 times the net gallonage to be moved per hour, and hard corals around 20 times or more. This movement is within the display aquarium, and does not apply to seawater moving through a sump.
The normal way of generating this movement is to employ power heads. More than one is usual, so that the outputs clash and chaotic and random movement is achieved. The output of the normal power head exits from a nozzle of, say, ½” (circa 12mm) in diameter. As this nozzle is narrow and the output is high, the seawater forms a powerful and narrow stream. This can be damaging to corals should it have a direct impact and care is needed in the coral’s placement. Even then corals in a good environment grow and may enter the high speed stream zone.
There is a relatively new type of power head now available which serves the same purpose as the type mentioned already. There is a very significant difference however, and this is that the nozzle is much wider, measuring around say 2½” to 3″ (circa 63mm to 75mm). Seawater leaving exits in a much gentler manner, which is not in a narrow stream but in a broader front. As the seawater stream is not so damaging the amount of seawater that is moved can be significantly increased over the older types of device, so the total amount of seawater that needs to be moved per hour is more easily achieved. The movement is more natural as well, as the stream forms, as said, a wider front. In the same way that the older power heads can be used in opposition, so can the wide output types. In fact, if two or more are used in opposition on timers then a wave system is not that difficult to generate.
The result of all this is generally happier corals. Provided the currents are not so strong that soft corals are being overcome by the surge and even hard corals are hard put to expand properly, the aquarist should see the corals are expanding and extending their polyps noticeably more.
The only time when the wide outlet power heads could need their output reducing is at feeding time to enable the fish to feed at leisure. Control units for the power heads, if used, normally have this ‘pause’ facility built in. Some aquarists also reduce the output of the power heads at night.
Anything that makes the aquarium environment a more natural one isn’t a bad thing, and the use of wide outlet power heads is a step in this direction.