Alot of the pumps people use really deteriorate fast when turned on and off every few hours. Even more so every 20 mins. Last thing you want is a pump that blew up in your tank while you were away.
Aren't their power heads that can be turned off and on frequently? Also, it seems like this method would use 2 pumps - one for each direction. If one stopped working, you would still have the other one until the broken one could be replaced.
What about one pump with a switch of some sort that directs the water down alternating pipes?
This is probably illegal - if so mods please delete it, but here is the article cut and pasted for those of you who don't want to go to a link. It's really not that long.
In recent years there has been a great amount of varied and original data produced on lighting and corals for the reef aquarium. By contrast, there have been much fewer articles on the effects and importance of water flow in the reef aquarium. Most aquarists are unaware of the relationship between laminar and turbulent flow and virtually no one ever discusses water motion for reef aquarium in terms which actually apply to fluid dynamics. It’s time for the reef aquarium hobby to catch up in the flow department. Just as aquarium lighting has received a thorough reevaluation, so too must we start to consider how the water movement of our aquariums translates into water motion which is relevant and suitable for coral health.
Describing the ideal flow
For lack of a better value, aquarists describe the amount of water motion in their reef aquariums in terms of turnover rate. If identical powerheads are placed at opposite ends of an aquarium but one powerhead is facing the center of the aquarium and the other powerhead is facing the aquarium glass, the powerhead which is directed towards the center of the aquarium will undoubtedly produce faster flow speeds and more circulation throughout the aquarium. Although the powerheads both have the same turnover rate their orientation to the main water mass has a great effect on the flow speeds they will produce. Since flow speed is the critical measure for determining the rate of gas exchange, turnover does little to convey how fast a coral will respire and photosynthesize.
In the natural environment, the reef surface and the corals which live on it experience mostly random, chaotic flow in the form of oscill
atory surge. In most cases, aquarists interpret “random, chaotic flow” to exclude laminar or unidirectional flow. This interpretation usually translates into a bevy of powerheads and inlets arranged to resemble what I call a squirt gun firing squad. Although reef aquariums are an attempt at recreating a natural environment, trying to reproduce surge with the scale and energy of the natural environment would take tremendous effort and resources. Whereas the oscillatory surge of the natural environment entails movement of the entire water mass, the typical aquarium features small plumes of water movement which lose velocity and momentum with distance away from the source of water flow (Harker 1998). Water flow which is exiting a powerhead or other outlet begins as high speed, unidirectional flow. However, as the flow increases distance from its source, resistance from other flows and viscous friction cause the orderly flow to quickly lose momentum. At this point the flow loses velocity and it increasingly becomes multidirectional, turbulent flow. Riddle illustrated this phenomenon using a digital electronic flow meter (Riddle 1996). The flow emanating directly from a Hagen 802 powerhead displayed velocities upwards of 70cm/s whereas the measured velocity was 0 cm/s only 60 cm away. This does not mean that there was no flow at that distance but rather it means that either the flow was not properly aligned with the flow meter or the sum of the multidirectional flows had a net velocity of 0cm/s. It is true that turbulence leads to an increased rate of mixing, but
fast laminar flow will become turbulent as soon as it encounters an irregular surface such as that of a coral. The faster the flow speed, the greater the amount of turbulence produced when the flow encounters a surface. Although turbulence is the desired end product of water movement, aquarists should be more focused on producing faster unidirectional flow.
The Flow Environment
The environment in which fluid movement occurs has a great effect on how the fluid will behave. The three things that will impact water movement in an aquarium are the dimensions of the aquarium, the relief of the live rock reef structure including the corals, and the force and duration of the water motion.
When considering an aquarium for use as a reef tank, it is important to remember that the dimensions of the reef tank will have a great influence on what kind of reef it will be. The size and shape of the tank will determine the type of lighting to be used, how maintenance will be performed, the type of fish and corals it can hold and how water flow will behave within the glass. As a stickler for water motion I usually consider the last criteria first. In a small aquarium, the water will be easy to flow throughout the entire aquarium, mass water movement will occur very quickly but there will not be much heterogeneity of flow speeds. In a smaller aquarium, viscous forces cause most of the water movement to move at a similar speed and it could be hard to provide suitably slow and fast water flow speeds for the corals which prefer one or the other. In a larger aquarium, it will take more force to move the entire water mass and it will take longer for the entire volume to circulate. However, once the entire water mass of a larger aquarium is moving, it will have more inertia and it will be less impeded by the reef structure or corals which project into mainstream flow. It is easier to produce a variety of flow regimes in larger tanks. Since a larger tank will be governed more by kinetic than viscous forces it is more likely to feature a narrow band of faster flow at the surface and a broader band of slower flow at the bottom.
As you might expect, the more live rock you have in an aquarium, the harder it will be to provide adequate water movement to the volume of the entire tank. Since I prefer to view corals from above, I generally use as little rock as possible. We have all heard the suggestion that a reef aquarium requires 1-2 pounds of live rock per gallon but since the density of live rock varies greatly, this is not a very useful guideline. As you can see in figure 1, my personal preference is to build a reef structure which occupies no more than about 25% of the actual volume of the aquarium. This restriction equates to a live rock arrangement which can be most of the length of the aquarium but it shouldn’t be much more than half as high and half a wide as the dimensions of the aquarium. Although new aquarists might be inclined to call this kind of set-up “empty,” a patient aquarist knows that this modest amount of rock leaves plenty of room for corals and growth.
In the ocean, surface currents of water are driven mostly by wind blowing across the surface of the sea. The amount of water moving in those currents is proportional to the force of the wind and the duration for which it blows, which is called the fetch. Since aquarists do not use wind to move water, for our aquariums we can think of the fetch as the duration that a mass of water is pushed in a particular direction. Commercially available “wavemakers” are not designed or constructed on anything more than the status quo of the coral hobby which is that corals and reef aquariums need “random, turbulent flow.” Apart from their high price, my biggest complaint about these pump controlling devices is that their outlets are switched on and off with such short intervals that they do not allow for an optimized fetch of water flow. By turning off a pump before it has had a chance to reach its full water movement potential, a water pump in this scenario essentially sends out a plume of water movement which encounters a lot of resistance from the inertia of the water volume. By increasing the duration that a water pump is turned on, the moving parcel of water will gain size and momentum so that when the pump is turned off, the water volume should continue to move through the aquarium for a short time. The capacity for wavemakers to produce mass water movement can be ameliorated by increasing the timing interval between pumps and designing pump circuits which work together to move the entire volume of the aquarium.
The final consideration for the flow environment is the placement of invertebrates in regions of the aquarium which combine suitable lighting intensities and water flow speeds. Since the upper region of the aquarium is often the preferred placement for high light corals, it is doubly advantageous to concentrate the fast water movement in the upper layers of the aquarium. Be mindful that when water flows around a shape, there is usually more turbulence and therefore more gas exchange on the downstream face of the shape. If you are looking at the upstream surface of a coral for indications of the coral’s behavioral response to water flow, you could be missing the more significant response on the downstream face of a coral. The bottom line is that you should do a colony wide inspection of your corals for indications of the suitability of the flow to which the coral is exposed.
