The water movement in a reef tank is quite possibly one of the most neglected areas of reef keeping. It is possibly the second most important factor to coral health, behind only lighting. Experienced reef aquarists spend a considerable amount of time trying to optimize flow in their tanks, but it is common for new comers in the hobby to ignore circulation altogether. Water movement provides needed gas exchange, brings nutrients, and sweeps detritus and waste products away from the corals. It also carries oxygen to the upper parts of the sand bed and through the live rock, maximizing their filtration capacity. Some corals benefit from a wave that blows them back and forth to increase the amount of surface area exposed to the light.
The quickest and easiest way to add water flow to a reef tank is by the use of pumps. It is possible to split the outlet of the pump to provide flow to various parts of the tank. Some aquarists construct spray bars out of PVC pipe to further expand the area of flow. While this is suitable for many applications, the flow provided from a static source such as this is a laminar flow.
If you have ever been swimming in the ocean, you'll immediately find that the surge is not like the output of a water pump. It is considerably stronger, and it varies in intensity and direction. While it may not be practical to mimic the intensity of an ocean current, it is important to make an attempt at some sort of wave current or oscillation. An oscillating current can be generated in a variety of ways:
1. The use of several power heads with a wave timer.
Advantage: Easy to setup and is flexible in the location of the pumps and the cycle at which they will operate.
Disadvantage: Most pumps are not designed for constant on/off use and may burn out faster than they normally would. Magnetic drive pumps upon starting make an audible clicking noise that can be distracting.
2. Dump Bucket Surge Device.
Advantage: Surge devices in general create a unique wave-form They send forth an intense surge of water that simulates a crashing wave. The surge is then followed by a calm period. The dump bucket is a beneficial design since it can be used as an algae filter if provided with a light source. Algae can be grown and harvested from the bucket's surface as a method of filtration.
Disadvantage: They are not easy to set up, often requiring a custom installation above the aquarium. The bucket itself can be difficult to maintain. The pivot the bucket rocks back and forth upon has a tendency to get stuck as debris accumulates. Also, the counterweights that draw the bucket back to its resting position may need to be adjusted occasionally as algae grows on the unit. The other serious drawback is the bubbles they send into the tank. The bubbles themselves are harmless, but when they reach the surface and pop, they spray the surrounding area with salt. That resultant salt creep is a maintenance nightmare to clean as it gets all over the lights, trim of the tank, and possibly the surrounding walls.
3. Carlson Surge Device and "Toilet Flapper" Surge Device
Both the toilet flapper design and the Carlson design use a large reservoir ranging anywhere from 5 - 100+ gallons to hold water. Once the reservoir fills to a certain level, the water quickly empties into the display aquarium.
Advantage: The advantage to these surge devices is the fact that they are simple and easily fabricated. Compared to a dump bucket that may cost a few hundred dollars to implement, a typical Carlson surge device can be made for less than fifty dollars. The toilet flapper design introduces less bubbles into the aquarium however has more moving parts than the Carlson surge device.
Disadvantage: While they are simple, they are not easy to set up, often requiring a custom installation above the aquarium. They are not quiet; in fact they may sound like a toilet bowl flushing (go figure). The other serious drawback is the bubbles they send into the tank. The bubbles themselves are harmless, but when they reach the surface and pop, they spray the surrounding area with salt. That resultant salt creep is a maintenance nightmare to clean as it gets all over the lights, trim of the tank, and possibly the surrounding walls.
4. Mechanized oscillator (Return Oscillation Device or Motorized Ball Valve)
Advantage: They provide an excellent, constantly changing flow pattern in the tank. They have a lower profile aesthetically than the other two options.
Disadvantage: They are expensive, in some cases, very expensive. The motors inside have a finite life span after which they will need to be replaced. The reliability of some models has been questioned; however, in my personal experience, they have never failed. a motorized ball valve will also require a controller to set its switching frequency.