Re-iterating The Importance Of Aquarium Water Movement

Seawater that is not moving or sluggish could be close to ‘dead’. In that condition it is not going to carry out the important functions that are necessary.

I remember once reading in a marine hobby book that the aquarist needn’t worry too much about fish and seawater movement, the fish swimming about would create enough movement. Er, sorry, but wrong! Admittedly the book was read many, many years ago and the author would be correct in one respect – as the fish swam along the seawater would move past them. This was written in the days well before corals had made an appearance, but nevertheless we now know better, a fish only aquarium would not be healthy without sufficient seawater movement.

So why is seawater movement so important? The movement supports important functions without which there would be problems.

The seawater needs to move fairly vigorously, not a maelstrom but significant. The movement should not be linear but random and chaotic.

In a fish only system the guideline is around 10 times the net gallonage of the display aquarium. Any gallonage in a sump is ignored. The first thing achieved is that oxygen in the seawater is plentiful. Oxygen is taken in at air/water interfaces, and the biggest one of these is the seawater surface in the display aquarium. Seawater is constantly moving to the surface and away again hopefully saturated with oxygen, which is then carried to all parts of the aquarium system including the sump if used. It can be seen how important sufficient oxygen intake is when it is considered that seawater in a marine aquarium with a high oxygen content is often between 6 and 8 ppm (parts per million). Not exactly a high ratio! In a heavily stocked fish only aquarium the demand for oxygen by the fish is high. Obviously a drop in the oxygen supply would cause fairly rapid symptoms such as fish close to the seawater surface where the oxygen content would be higher. The second advantage to fish is that the seawater movement prevents an ‘envelope’ of static seawater from developing around the fish, which, as I understand it, could interfere with the fish’s osmotic requirements. The movement, on a much more secondary level, makes food move about attracting the fish and inviting them to chase it down.

In a soft coral reef aquarium, fish or no fish, the guideline for seawater movement is the same as in the fish only system which is about 10 times the gallonage of the display aquarium per hour. The seawater is oxygenated in the same way, and the movement brings this oxygen to all parts of the system and around the corals. Branched soft corals can be seen responding to the movement as a field of corn does to wind, and the corals are likely to extend their polyps further than in a system with poor circulation. Some corals would not exhibit their polyps at all. The seawater movement helps the corals clear mucus and debris from their surfaces and also brings food within reach.

Generally SPS (soft coral stony) hard corals require more movement than the soft variety. The guideline is around 20 times the net gallonage of the display aquarium. This seawater movement gives the same advantages to the hard corals as it does to the soft types – it causes better polyp extension, clears the surface of debris and mucus, and brings food. The reason more vigorous movement is required is because of the area the corals inhabit on the reef.

When an aquarist is considering purchasing a coral, consideration should be given to the area on the wild reef the coral would normally inhabit. This will permit better placing of the coral on the captive reef in relation to light and seawater movement. It is quite possible for an aquarist with a hard coral reef system where seawater movement is around 20+ times per hour to keep soft corals (subject to compatibility) – the guidelines are not that rigorous. The seawater movement lower down is often more subdued and suitable for soft corals.

When all is considered, there aren’t that many demands that have to be met for a healthy marine aquarium. Top of the list is seawater quality. This includes oxygen content which means sufficient and appropriate seawater movement is required.


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  1. I didn’t do it on purpose, honest!

    I’ve noted that the abbreviation SPS means ‘soft coral stony.’ This of course is incorrect, it should be ‘small polyp stony.’

    Just can’t get the staff!

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