Water flow is needed in the wild and in our aquariums to keep most of our aquatic animals alive. Not only does water flow oxygenate the water, it brings food to organisms such as corals, stimulates growth and keeps sessile animals free of sediment.
We take cues from the wild for our aquariums. If a coral lives in a high-energy environment in the wild, it is best to provide it with a high-energy environment in the aquarium. There are ways that you can provide adequate water flow for your aquatic animals, depending on your setup.
How Oxygen Gets into Water
Diffusion. If you ask most people how oxygen gets into water, they would tell you that one way is by diffusion. And they would be correct. Diffusion is the movement of a substance, such as oxygen, from an area of higher concentration to an area of lower concentration. The challenge is that diffusion happens at a molecular level and is very slow. How slow? In a series of lectures on the chemical features of water, M.N. Kutty (1987) explained how the solubility of oxygen in water is so small and the diffusion process so slow that it would take six years for oxygen to diffuse from the surface to a depth of 20 feet in still water. Almost all the dissolved oxygen diffusion in natural waters is assisted by some form of water motion.
Aquatic animals that rely on diffusion for respiration tend to be very small or inactive. Some of the larger, more active animals that depend on diffusion are the flatworms. These animals are literally as thin as a sheet of paper. This gives them a high surface-area-to-volume ratio. Larger, more active animals have circulatory systems, hearts to pump blood and gills or lungs to exchange CO2 for oxygen.
Plants and algae. Another answer to the question of how oxygen gets into seawater is that it is produced by underwater plants and algae. About half of the oxygen we breathe is produced in the oceans. But the production of oxygen by photosynthesis is limited to the photic zone, a small portion of the world’s oceans.
Water flow. Neither diffusion nor photosynthesis explains why there is oxygen throughout most of the ocean and in many of the deepest lakes. The circulation of water is critical for the Earth’s aquatic ecosystems and also in aquariums and ponds.
In Our Aquariums
There are animals like coral polyps and the flatworms mentioned earlier that do not have a circulatory system and rely on diffusion (at least internally). These animals tend to be small and slow-moving. Similarly, it is possible to keep an aquarium without water circulation. The original parlor aquariums of the Victorian era relied on diffusion and plants to provide enough oxygen. Breeders of small killifish often keep small breeding groups in aquariums with just plants. Some fish, such as bettas, are adapted to low-oxygen environments and can obtain oxygen by gulping air at the surface. However, animals and aquatic systems that lack circulation are generally limited in size and capacity.
Most modern aquariums have high biological loads that require good circulation. Although corals rely on diffusion, they often live in high-energy environments where currents, turbulence and wave surges commonly provide water movement around them. Perhaps because they internally rely on diffusion, captive corals are especially sensitive to water movement. In nature, the forms that corals take can be highly variable — the same species may look very different in a protected lagoon than it does growing on a high-energy rim reef.
For most aquariums and ponds, it is important to have good water circulation throughout the areas in the aquarium where aerobic life, such as fish, will be. This will ensure that these areas have sufficient oxygen. There are many ways to circulate water — air pumps, powerheads and filters are commonly used in modern aquariums. There are also special submersible pumps that are specifically designed to efficiently increase water circulation. Particles floating in the water can be observed to get an idea of what the aquarium’s internal water flow patterns are. It is also a good idea to ensure that water from the bottom of the tank or pond is making its way to the top and vice-versa.
The amount of water flow needed depends on what kind and amount of life the system is supporting. For most systems, water that is noticeably and gently moving but not moving fast enough to stress the aquarium’s inhabitants is a good goal.
Carlson Surge Devices
In the 1980s and 90s, under then-Director Bruce Carlson, Ph.D., and others such as Charles Delbeek, the Waikiki Aquarium earned a reputation for excellence in stony coral propagation. One of the inventions from that productive era is the “Carlson Surge Device.” Bruce Carlson went on to work at the Georgia Aquarium, which he helped design. He recently retired to Hawaii, where I talked with him about the Carlson Surge Device.
Applications for the Device
The surge device is best for tanks 500 to 5,000 gallons in size, especially those that are outdoors, in commercial facilities or in public aquariums. Carlson warns that scaled-down versions are finicky, and all sizes produce a lot of bubbles, which are not good inside a residential environment (the salt from the bubbles will get into the air and cause corrosion in the house). For really big tanks, the engineering becomes much harder, and variable-drive water pumps are a better solution. For home reef aquariums, the best solution is propeller-style water pumps coupled with electronic controllers.
How a Surge Device is Helpful
Corals need strong periodic surge and turbulence. The surge device provides a lot of flow cheaply; other than the pump, there are no moving parts. The Edge of the Reef tank at the Waikiki Aquarium uses the same surge device installed in the 1980s. Its upkeep is minimal and consists of occasionally replacing the pump.
What the Device Does and How it Works
The device provides an intermittent turbulent surge of water into reef tanks. The duration of the surge depends on the size of the container used and the size of the siphon used. The frequency of the surges also depends on the power of the system’s pump. The pump runs constantly. Some surge systems cycle pumps on and off, which can prematurely age the pumps.
The pump fills a reservoir above the tank. Once the reservoir reaches a predetermined level, a siphon forms and drains the water from the reservoir back into the display tank. Once the reservoir is mostly drained, the siphon breaks, and the reservoir beings to fill again. The end of the siphon in the display tank needs to be underwater for the system to work best. The siphon needs to drain the reservoir faster than the pump fills it.
If the siphon fails to start, the reservoir has an overflow built into it. The system can be noisy, but there are tricks to mitigate this, such as placing the reservoir on foam board, putting a lid on the reservoir and cutting the siphon tube in the reservoir at an angle.
Some specialized systems require specific water flow patterns. Jellyfish and other plankton are best-kept in kreisel aquariums, which have a specialized gentle laminar water flow that keeps the animals suspended and away from the sides of the tank. On the opposite extreme, a friend of mine was conducting research on an endangered species of freshwater goby that lived in mountain streams with a high flow rate. He used an electric outboard boat engine to maintain a high water flow rate in a special oval raceway aquarium with plenty of rocks and turbulence.
Reef tanks require more attention to water flow than most systems. Both light levels and water flow patterns are critical variables required for keeping healthy reefbuilding corals. Some soft corals, such as sea fans, utilize the constant currents. However, too much of the same thing can harm some corals. Most corals benefit from periodic surge and turbulence. Surge and turbulence can be accomplished through the use of dump buckets, Carlson Surge Towers (see “Carlson Surge Devices” sidebar) or submersible propeller-bladed water pumps placed on timers. Water flow not only aids in gas exchange, but it stimulates growth, brings food and helps keep corals free of sediment.
So take a look at your aquarium’s water flow and make sure that there are no dead zones. Provide surge and turbulence for animals like corals that need it, or turn down the water flow for animals used to calmer waters. Take a cue from nature and replicate water flow for your animals just as you would with diet, light, decor, etc. And remember that sometimes with aquariums, as with life, it’s just best to go with the flow. AFI
James B. Wood, Ph.D., is a marine biologist, educator, underwater photographer, webmaster and author. He was recently appointed as an Affiliate Faculty member at the Hawaii Institute of Marine Biology and currently lives in South Florida.