Water circulation within a reef aquarium is important for many reasons. It allows for food to be provided to corals, waste to be removed from corals, oxygenation of the water at the air interface, delivery of water to live rock as well as making the aquarium a more natural habitat for the aquarium inhabitants.
The easiest way to describe closed loop is using an external filter as an example. With an external filter there is in inlet pipe which delivers water to the filter. The filter pump then pushes the water through the filter and it is delivered back to the aquarium via a single outlet.
Closed loop circulation is exactly the same as this with the exceptions that there is no filtration involved and the water velocity is greater.
Sounds very simple and in reality it is. With a closed loop the water inlet to the pump is located underwater as is the outlet so effectively you have a ‘closed loop’ of pipe work which is connected to an external pump.
The inlet, pump and outlet when connected together are known as a loop
In this type of system normally one pump is used per loop however one loop can contain numerous outlets. For example the inlet is under water in the aquarium. Water is provided to the pump which, when turned on pushed water back up into the aquarium. As the pumps output is connected to pipe work this is run up to the aquarium and then inside the aquarium.
Once in the aquarium there is a run of pipe which can be located anywhere in the aquarium as long as the outlet(s) are under water. It is normally recommended that a loop has no more than 3 outlets however it does need to be noted that the more outlets the loop has the more the power of the water force will reduce.
Therefore from one loop you could have three outlets which can be located in different areas of the aquarium, therefore providing more areas of flow from one pump.
One consideration is the physical size of the aquarium. The bigger the aquarium the more water there is and therefore more water to move around.
The first thing you need to consider is the physical power of the pump. The pumps are rated at zero head height with only one outlet. If you only plan on using this outlet then this will be the output you can expect from the pump. There are no head height restrictions in a closed loop system but I will go over that shortly.
The second thing to plan for is the amount of outlets per loop. It is recommended that you do not go above three outlets per loop as the reduction in flow may be too much. Pumps are rated in accordance with the size of the outlet coming out of the pump and with a pump there is only one outlet. If you put three outlets onto a closed loop then effectively the output from the pump per outlet will reduce. Normally in a loop this reduction is staged with the last outlet having the weakest route.
The physical plumbing is also an area which requires consideration. With a closed loop system you will have one pipe for the inlet and another for the outlet. With this plumbing running down to where the pump is located there may be a fair few pipe, especially if you install more than one loop! The run of these pipes will need to be roughly planned for and it is wise to ensure that they are located in an area where they are accessible. If a leak was to occur then you want to be able to get to the pipe work to rectify the issue.
Once everything is in place the next step is to get water into the pipes and all the air removed. If you have drilled holes in the aquarium for the inlet/outlet(s)s then water will already be in the pipe and as soon as the pump is activated any remaining air should be pushed out. To check if there is any remaining in the pump simply rock the pump gently to remove any trapped air bubbles.
A benefit of closed loops is that a very un-natural looking device is removed from the aquarium. Power heads etc are very good devices at moving water around however they are, in my opinion, quite intrusive. With a closed loop system the physical device is removed from the aquarium, out of view with only the outlets being visible.
With the pumps being external to the aquarium a heat source is removed from the aquarium. Whilst this heat source may be valuable during the colder months it is certainly an issue during the warmer months. With the heat creating source being physically removed it can, at times be easier to provide stability in relation to temperature.
A very valuable benefit of a closed loop system is that there are no head height restrictions. The rating of the pump is what you should get – even if the pump is three feet lower than the display aquarium. Head height is effectively due to gravity where the water when rising up a tube has to fight against this natural force. The pump can only pump the water so high and then simply runs out of power.
If a pump has a four foot head height then at four foot above the pump the water flow will stop, at two foot above the pump the flow will be halved etc. With a closed loop aquarium head height does not exist. The reason for this is that the water does not have to fight against gravity as no air is allowed into the pipe work. The inlet pipe is continuously full of water as is the outlet therefore with no gravity to fight against you get full use of the power of the pump!
The first consideration is that of water flow and friction. Water will always find the easiest route and follow it which is one of the reasons you need to plan your plumbing. If you implement a tee piece for example the water in the pipe will travel down the easiest route and therefore water flow down the secondary path may be severely impacted. Friction is also the same – although the inside of the pipe feels smooth to the touch it will create friction when in contact with the pipe.
When in a straight line this friction is not a problem however bends are another kettle of fish altogether. If you need to go around a bend it is easy to install a 90 degree bend however this is quite a bend for the water to go around and the flow will be severely impacted as well as creating back pressure within the pipe which over time may damage the pump itself. If you need to go around a corner then it is better to use two 45 degree bends instead of a single 90 degree bend.