Why The Emphasis On Seawater Movement?

Seawater quality is a high priority in a marine system, and without it the aquarist is going to have niggles and problems. Maintaining seawater quality nowadays is easier with all the technical support equipment available, for example the protein skimmer and the high quality dry salt mixes that are commercially produced.

Even with seawater mixed properly and equipment fitted correctly, the quality story doesn’t end there. There is another important requirement and that is adequate seawater movement. Without this the aquarist will still likely be faced with niggles and problems. It could be argued that movement is a part of seawater quality as it contributes to it so significantly.

Before going further a mention of the guidelines for movement should be made. Note that these are guidelines and not rules. They provide a basis which could be altered if the need arose. In a fish only aquarium seawater movement should be around 10 times the net gallonage in the display aquarium. In a soft coral system the same guideline applies. In a hard coral (SPS) system the movement should be around 20 times the net gallonage in the display aquarium. Note that these guidelines are for the display aquarium only, if a sump is used this is not included in the gallonage calculation.

With adequate movement any temperature differentials will be minimized, and the heater’s controlling thermostat will more likely read the average temperature and react correctly. A temperature differential could occur for example in an area where there is very sluggish movement as the seawater enters and exits the area very slowly. If there is adequate seawater movement overall then as said the heater thermostat will read the overall temperature more correctly. There are always areas in a system where the seawater flow is lower, for example in and under reef rockwork or in the corners of the aquarium. It is sluggish flow that needs to be avoided.

Fish will be healthy and more settled, all things being equal, where there is good movement. It is reported that in sluggish or still seawater fish could be surrounded by a thin ‘dead’ area that interferes with their osmosis needs. In any case, on the wild reef the seawater isn’t usually sluggish and life adapts to its normal environment over a long period of time.

Where there is a reef with soft and/or hard corals, movement is very important. Corals rely on movement to bring food to them, and at the same time remove mucus and dirt. Coral extension should be better with adequate movement and, again all things being equal, growth and colour should be good.

The following is probably the most important aspect of ensuring that seawater movement is as it should be. There is an ongoing requirement in a marine system for stability and there should be nothing that could affect this.

Gas exchange is often mentioned in relation to marine systems, and this simply means the exchange of gases at air/water interfaces. Unwanted gas escapes and another, oxygen, is taken in. Air/water interfaces are the seawater surface in the display aquarium and also in the sump if one is used which are the most important, and also seawater flowing over weirs and down overflow pipes. Some equipment presents an air/water interface, for example the protein skimmer.

If there is to be adequate gas exchange it follows that the seawater must continually move to the surface and away again. Where movement is adequate this happens and the seawater from all parts of the aquarium meets an air/water interface. Movement is often more vigorous in the upper areas of the aquarium but this doesn’t matter provided the lower areas are moved reasonably also.

Everyone knows that life forms require oxygen, including humans. Fish etc have less available to breathe than humans – there is about twenty times more in the air than there is in warm seawater. Cold fresh water holds more oxygen than warm fresh water, and warm fresh water holds more than cold seawater. As the seawater temperature increases, so there is less oxygen. In a reef aquarium that is well designed and maintained the oxygen content is around 6 to 8 ppm (yes, that’s right, about 6 to 8 parts per million! Not a great deal). The fish and other life have a constant demand for oxygen and so it must be constantly replenished or, particularly in a heavily stocked fish only aquarium, there will be problems. An example of how a dangerous state can arise with oxygen levels is where a lengthy power cut occurs and all seawater movement has stopped. The fish etc continue to breathe of course, and eventually the oxygen content of the seawater drops very low. The fish will eventually probably come to the surface and gasp as there is some remaining oxygen in that area, they might even extend their mouths above the surface in a desperate attempt to breathe. If the seawater circulation returns in time the situation will remedy itself and all should return to normal.

Good seawater movement is going to provide adequate oxygen, which will be moved to all areas of the aquarium ensuring that all life has a supply. One very essential area that needs the oxygen is the bio-filter. Those hardworking friends of the aquarist, the bacteria, are oxygen hungry and must have an adequate supply to function (those that convert the toxins anyway).

Talking of bacteria and bio-filters, aquarists using live rock have the advantage that the whole of the nitrogen cycle should be achieved, which are the conversion of toxins and the reduction of nitrate. When the bacteria remove oxygen from the nitrate so breaking the nitrate down, the result is released as gaseous nitrogen. It is released at air/water interfaces so again circulation is important.

There isn’t any intention of delving into chemistry as there isn’t a need here, but seawater could be adversely affected if there is poor circulation – as gases cannot easily be released from the seawater pressure increases for the seawater to react in an unwanted way. There could be a reduction in pH for example. Stability is required and good seawater movement goes a long way to provide it.

Creating seawater movement is straightforward and could only need for example two powerheads, though the need will vary from system to system. Linear seawater movement is not required and a little experimentation with positioning the powerheads should produce chaotic or more random movement, which is required. Whatever method to move the seawater is used, the livestock will demonstrate their appreciation with better health and colour.


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