We aquarists face many challenges re-creating natural aquatic habitats, but one stands out from the others: creating realistic water motion in our closed systems. The key role that water motion plays in the health of aquatic animals is not as well understood as many other aspects of the hobby.
Yet the role that water motion plays in maintaining a healthy environment is a complex one. Moving water is richer in oxygen than still water, so water motion aids in gas exchange. Flowing water brings dissolved and particulate nutrients to sessile (immobile) animals, such as mollusks, sponges, tunicates and corals. Water motion carries away waste products that would otherwise accumulate and degrade water quality.
Virtually all aquatic ecosystems are continually bathed in constantly changing water as it flows across the area. Tidal currents, winds and wave action constantly churn the waters surrounding a coral reef. Freshwater habitats are also continually flushed as freshwater flows toward the sea.
Finding the Right Water Movement
While moving water is essential for the health of every aquatic habitat, water motion of the wrong kind can create problems, as well. Many aquatic organisms can be stressed and even damaged by excessively strong water motion. The message for aquarists is that we need to create realistic water motion that is both natural and adequate, while also realizing that there is actually danger in creating the wrong kind of water movement.
Finding that balance turns out to be harder than you might think. The traditional approach to water motion in an aquarium is to position one or more small powerheads at the surface in the corners of the aquarium. This churns the top of the water and gives the impression of a lot of water motion. In a fish-only aquarium, this approach is quite acceptable. Water motion at the top of the aquarium aids gas exchange, and this is the primary need in a fish aquarium. The only downside is that limiting water motion to the surface area allows solid waste to accumulate at the bottom. Fortunately, periodic vacuuming can take care of that.
Yet the more complex an aquarium’s eco-system, the less the corner-powerhead approach creates a satisfactory solution. The larger the aquarium or the more densely it is aquascaped, the harder it is to have adequate water motion throughout the aquarium. For example, in densely planted freshwater aquariums, surface water motion does not penetrate more than a few inches below the surface. The denser the growth, the more important it is to move water from the bottom of the aquarium to the surface. One solution is to add traditional airstone-powered lift tubes to the aquarium. These move water up from the bottom of the aquarium to the top. Combine the lifts with powerheads at the surface, and you will have a low-cost solution for all but the largest planted aquariums.
Reef aquariums need even more water motion than planted freshwater systems. Many traditional reef aquarium inhabitants are invertebrates that require a constant flow of water over their surfaces. Inadequate water motion can stress them, and in extreme cases, it can literally suffocate the animals. This is why a reef hobbyist needs to take special care in creating a water-motion regimen. One common approach to increase the amount of water movement is to use more powerful powerheads. Powerful powerheads have larger impellers that push more water. The problem is that larger powerheads move more water by creating a higher-velocity water stream; the danger is that high-velocity water flowing over an invertebrate can damage it.
Sessile invertebrates can’t find a better place to live. They have to live wherever you’ve placed them in the aquarium. If you place them near the stream from a powerful powerhead, they could be damaged.
Replicate Natural Flow
The water motion found over natural aquatic ecosystems is diffused and turbulent. There is a great deal of energy involved, but the force of the water flow is not focused like if it is coming out of a powerhead. The velocity of water in any given direction is small, but the total movement of water in all directions is high. To mimic the water flow across a natural coral reef, we want large volumes of water flowing at a low-velocity flow.
Think of the difference between the water flowing out of a garden hose and one from a high-pressure power washer. Because of the larger cross-section of a garden hose, the volume of water flowing from a hose may actually be greater than the flow from a power washer, but the velocity of the water is lower. How do we create the flow from a garden hose rather than a power washer?
We can achieve high volume, low-velocity water flow in a number of ways. One solution is to use a number of small powerheads rather than one large powerhead. Four small powerheads will create more natural turbulence than one or two large powerheads. Because the volume of water moving is a function of the size of the powerhead, and velocity is related to the cross-section of a powerhead’s nozzle, using multiple small powerheads creates more natural water movement. To create even more turbulence, the powerheads can be randomly switched on and off using a wavemaker.
While this approach is an improvement, the majority of turbulence is still at the surface, not lower in the aquarium where it is needed. To create water motion lower in the aquarium, one can mount powerheads lower. For aesthetic reasons, most hobbyists avoid this, so a more aesthetic solution is to use one or more closed loops for circulation lower in the aquarium. A closed circulation loop draws water from one portion of the aquarium and (by an external pump) returns it to the aquarium somewhere else. This approach is particularly effective in long, narrow aquariums.
A closed-loop system can use a large pump and move a tremendous volume of water at low velocity by also returning the water using a large cross-section manifold. A manifold is simply any device that has one inlet and multiple outlets. The hobbyist controls the velocity of water exiting the closed loop by increasing the number of outlets until the velocity of water is slow, but the volume remains high.
A relatively new development in aquarium water motion is the propeller pump. A propeller pump uses an electric motor to turn a propeller. The high-velocity flow of a powerhead is created by an impeller pushing water through a nozzle. The propeller pump does away with the nozzle. The propeller pushes water in all directions, so the dangerous shearing effect of high-velocity water is quickly dissipated.
All of these methods employ pumps that run continuously. There are other greener methods that use less power. Approaches that use periodic surges of water, such as dump buckets, can create very realistic water movement. They do tend to demand a lot of space and are high-maintenance.
Zonation and Water Movement
Coral reef zonation divides the coral reef into areas based on environmental factors, particularly water motion. The zones in which animals are found is a useful guide in determining how much water motion an animal likes in the wild.
Fore reef. The fore reef is exposed to storms, strong surges and high waves. The animals that survive on the fore reef tend to be fish that thrive in the surge zone and strong hard corals. Stubby branch-ing corals, along with boulder and encrusting corals, are the most common. There tends to be little sand in this zone, so fish that swim in the open water outnumber more cryptic saltwater fish that burrow or use coral cover for protection.
Anemones like turbulent water but are sensitive to shearing forces that can tear tissue. Anemones occur with their commensal clownfish in somewhat protected areas of the fore reef where grooves have been carved out from the rock.
Back reef. The fore reef takes the brunt of storms, so the back reef tends to be more protected and calmer. This is the area of greatest diversity because most animals can live in this environment. Stony corals predominate, but these corals tend to be larger and grow more quickly than those on the fore reef.
Clams and other mollusks can be found here, along with schooling fish, such as damsels and anthias, that use branching corals for protection.
Patch reef. The calmest waters are found around patch reefs and lagoons. Soft corals mix with fragile stony corals (so-called bottlebrush corals). Patch reefs accumulate sand, driven landward by storms along with detritus, so the bottom is a mix of mud and sand, drawing bottom-dwelling animals that burrow in the sand and live on the food they can filter from the substrate. Patches of algae and sea grass can often be found around the patch reef.
How a Closed-Loop System Can Improve Circulation
The primary reason water is typically returned from the sump to the aquarium near the surface is to prevent back-siphoning in case of a loss of power. Water can be returned in the lower portion of the aquarium by employing a closed-loop circulation system (CLCS). A CLCS increases circulation throughout the aquarium by drawing water from near the surface of the water and then pumping it through a closed loop to an outlet placed lower in the aquarium. Because the loop is closed, the water level in the aquarium will remain at a safe level even if there is a loss of power.
If you are contemplating a very large aquarium, using a surge method can save a lot of money, but for the majority of hobbyists, the methods outlined in this article can achieve equal success with a lot less trouble.