Quantifying Flow Rates in the Reef Aquarium and the Requirement of One Acropora Species

This entry was posted on Monday, July 29th, 1996 and is filed under Journal of MaquaCulture.

Article by Dana Riddle, Copyright 1996. Reproduced by permission.
(This article first appeared in Volume 4 Issue 3, The Journal of MaquaCulture 1996)

Success with a reef aquarium depends on many factors. It is generally acknowledged that lighting and water movement are two of the more important parameters to consider. Advanced hobbyists can measure light intensity with lux or quantum meters.

The light and quantum measurements can often exceed that of the sun under some of the higher wattage lighting systems. It is only a matter of proper specimen placement to achieve the desired result…, but what of water movement?

Little has been published in hobbyist literature concerning proper water motion. Further, we are only now beginning to understand the water velocity requirements of different corals. The popular terms – “low” or “high” water movement are too subjective to be of any real use. This short article will discuss quantified water velocities in nature and the reef aquarium.

Natural coral habitats are often described as “exposed”, “semi-exposed” or “sheltered”. These terms describe the varying degrees of wave action in a given area. For instance; “exposed” means there is strong wave action, while “sheltered” means the water is relatively calm. It is not surprising that different groups of corals inhabit these extremes. By knowing what to look for, the hobbyist can determine the amount of water movement required by the animal. While a full review of all corals is beyond the scope of this article, it is an important concept for the hobbyist to understand.

In nature, there is obviously a range of water velocities. Generally, lagoon currents are about 20 to 30 centimeters per second (cm/sec). Reef flats may experience velocities of 60 cm/sec with the changing tides. Surge channels have been measured with velocities of up to 390 cm/sec. It is understandable that researchers are reluctant to gather data at extreme velocity sites, therefore, we can only guess as to the water motion found at some exposed sites.

In the aquarium, small pumps are usually employed to provide water circulation. “Power heads” are quite popular; these centrifugal pumps were originally developed for use in conjunction with undergravel filters. When these filters fell into disfavor with reef hobbyists, the manufacturers cleverly advertised them as providers of water circulation and they are still considered the standard for many hobbyists.

In an attempt to provide “good” flow in the aquarium, two or more Power heads are often used. It is tempting to believe that properly placed pumps will create a circular movement in the aquarium. Employing “wavemakers” to alternate pumps, the water motion is reversed and all is “well”.

While it is certainly possible to provide proper currents in this manner, there are many cases that will not be adequate. Power heads, when hung from the side of the aquarium and near the top, will create localized water movement. However, inadequate movement for many small-polyped stony (SPS) corals will not be found near the bottom of the aquarium.

Recommendations are often made by manufacturers based on their best estimates of their product’s practicality for certain tank sizes. These do not seem to be based on any original research; instead they seem to be a “seat-of-the-pants” estimation. They do not take into account the types of animals to be maintained. We are beginning to understand the requirements of individual animals.


Distance (in inches) from nozzle 0″* 2″ 4″ 6″ 8″ 10″ 12″ 14″ 16″ 24″
ft/sec 2.28 2.24 1.96 1.78 1.38 1.04 0.92 0.59 0.45 0.12
Table 1: Measured velocities.
* measured at nozzle

To assess the flow requirements of different corals, I have used a digital water velocity meter made by Marsh-McBirney, Inc., Frederick, Maryland. In one of many tests, the water velocity produced by a Hagen 802 was measured. Velocity attenuation (amount, force or value) was also measured. The results are reported in table 1.Velocities produced by various Power heads (and other pumps) vary; some produce much less, others produce much more.

Water Velocity Requirement of an Acropora Colony

Water movement must be of sufficient velocity to create an area of low pressure over a coral head. Water speed picks up as it takes the path of least resistance over the coral. The coral head acts as a brake to the water entering the spaces between branches and an area of high pressure is created. The water movement is towards the area of low pressure and as long as velocity is sufficient, water will be pulled upwards through the coral.

In a sense, the coral head is somewhat like an airplane wing. Without good low pressure, the coral will “crash”. In order to judge water velocity and its effect on different coral shapes, I constructed a flow chamber made of sheet PVC plastic. An acrylic window allows me to observe the impact of varying water velocities. (Refer to diagram 1) Water is pumped from a sump by a Little Giant MD4SC to the chamber which overflows back to the sump. Flow is regulated by a ball valve on the pump’s discharge. A thin layer of detritus (vacuumed from a live sand bed) is applied to the coral heads. When the flow is “saturating”, the detritus is lifted away from the coral. It is amazing to watch – there will be some removal of detritus from the leading edge of the coral, but when saturation is reached, the detritus, for want of a better word, “dances” away. In the case of one Acropora colony , Acropora gemmifera (?) 8 inches in diameter, saturation was reached at approximately 0.8 feet per second. It is certain that different shaped and sized coral heads will require different flows. When we compare the flow saturation figure with the power head velocities, we can quickly see this coral will need to be positioned in a rather narrow range. Placed too close, living tissue can be ripped away from the skeleton; too far away, the animal will suffer when detritus collects between the branches.

Diagram 1: Coral Flow Saturation Studies
(Not available at this time)

We often hear tips to use a kitchen-type baster to blow collected detritus from the crevices of captive corals.

This recommendation addresses the problem but not the cause!


  1. Andrews, J.C., Gay, S. and P.W. Sammarco, 1988. Influence of circulation on self-seeding patterns at Helix reef – Great Barrier Reef.
  2. Proceedings 6th International Coral Reef Symposium, Australia. Riddle, D., 1995, The Captive Reef: A Concise Guide to Reef Aquaria in the Home, Energy Savers Unlimited, Inc., Harbor City, CA 297 p.
Breeder's Registry staff are all volunteers and appreciate any comments or corrections on any of our posts.

Leave a Reply

Your email address will not be published. Required fields are marked *