Closed Loop Water Circulation

As with many things in this hobby closed loop circulation is a subject matter which can be quite confusing. This topic has been briefly covered before however in this article I hope to cover this area in more detail.

Water circulation in an aquarium, especially a reef aquarium where closed loop circulation is more suited for is exceptionally beneficial. I rate water circulation as third in a list of must haves – the two above it are water quality and lighting. Water circulation by many is actually classed within the water quality area however I prefer to keep it separate.

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.

There are many ways for the aquarists to provide water circulation of which the closed loop method is just one.

So what is closed loop circulation, what are the benefits and how does it work?

The easiest way to describe closed loop is using an external canister filter as an example. With an external canister file there is in inlet pipe which delivers water to the filter. The canister 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 that the water velocity is greater.

Sounds very simple and in reality it is. With a closed loop the water inlet to the pump (which is located externally to the aquarium) 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

What happens is that water flows down the inlet to the water pump. When the pump is switched on water is forced up the outlet pipe and out into the aquarium. With water being pushed up the pipe water is drawn down the inlet pipe and the cycle continues.

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.

So why would anyone consider using more than one loop? This depends realistically upon the size of the aquarium and types of corals being kept in the aquarium. If you keep short polyp stony (SPS) corals then these require stronger water circulation therefore with no closed loop devices you will either require stronger devices or more of them.

The other consideration is the physical size of the aquarium. The bigger the aquarium the more water there is and therefore more water to move around.  As with the above example in relation to SPS corals you can therefore either purchase more devices or purchase more powerful ones.

With more than one loop in the aquarium you can provide more outlets to where it is required most. For example if you have a 4*2*2 aquarium which is stocked with SPS corals then you could use stream like devices which do create excellent water circulation however these would be visible within the aquarium or you could create some loops.

In the above example what you could do with a closed loop system is install two loops. Both of these loops would have their dedicated inlet as well as a dedicated pump however you could feasibly have up to six outlets.  Once the pipe work is plumbed back from the pump to the aquarium these could be located in numerous areas of the aquarium. You could have two outlets at the corners in the front bottom of the aquarium pushing water up the rock face, another two could be in the rear upper corner and another couple could be hidden in the rock face. All the pipe work could be hidden from view either within/behind the rockwork or under the sand. All that would be visible would be the outlets and these would become covered in coralline very quickly.

The water flow created by a properly designed closed loop system can be fantastic however there is some planning to do.

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 is 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 – I will cover why this is shortly.

The location of outlets is an important thing to plan for. You don’t want to install your rocks and then plumb around them. It is much easier to attempt to design where you would like the outlets to be. Of course when you aquascape the aquarium this design may have to change a little however it should not need to change that much.

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.

Plumbing is an important aspect to this as there are many ways to ‘plumb in’ a loop. The connections to the pump are fairly straight forward. Either the pipe will attach directly to the inlet/outlet area of the pump or if not you simply attach some flexible tubing to the inlet/outlet, heat up the other end and stretch it over the pipe using bushes if required. It is the inlet and outlet which is important. There are effectively two ways in which the inlet and outlet can be integrated with the aquarium. The first is that the pipe work goes up and over the aquarium edge. Effectively pipe work is run up the exterior of the aquarium and then using bends it is taken over the edge and into the aquarium. On the inlet it is best to implement some type of protective cover to prevent livestock etc being drawn into the pump. The outlet is simply taken over into the aquarium and the outlets plumbed in.

The other way to achieve this is to physically drill holes in the aquarium. One of these would be for the inlet (again covered with a protective covering). In this hole would be a tank connector which the pipe work would be attached to and connected down to the pump. The outlets could either be just the one hole of one for each required outlet. With one hole you could just use it as a single outlet or you could plumb pipe work internally to more outlets. With more than one drilled hole you can split the pipe externally to the aquarium and deliver one pipe to each hole. With all holes you will need to install a tank connector and then connect the pipe to this.

Plumbing the loop(s) in is quite a simple process. It has been briefly covered above how to attach the pipe work to the pump however one area which I would recommend is locating the pump on some sort of media/device which will absorb vibrations. The pump, due to it having moving components is going to vibrate and due to hard pipes being attached to it may sometimes cause noise. If you locate it on some media then this noise will reduce. I have had numerous closed loops and have not been able to hear the noise of the pump. Due to vibration I personally prefer to attach flexible tubing to the pump and then attach the hard pipe to the flexible tube. This method therefore allows for a certain amount of give.

Take your time when plumbing the aquarium. As the old saying goes ‘measure twice and cut once’. You need to ensure that the pipe work you purchase is of food grade quality. Fortunately most quality online and offline shops now stock various types of plumbing components. You will also need some ‘glue’ for connecting the pipes together – this is not just normal household glue it is special glue which again you can get from your local fish shop or reputable do it yourself store.

When you cut the pipe always ensure that the ends have a straight cut. Once the cut is made ensure that any loose areas are moved and/or cleaned and then de-burr the end. You will require connectors to attach the pipes together. Considerations into these are given later. I would recommend not rushing this point and always dry test each pipe/connector. Once you are happy with the connection glue them together. Once the glue is set the only way you are going to be able to get them apart again is by cutting them!

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.

If the loops are configured in an up and over fashion then this is slightly trickier to accomplish as the water has not ability of getting up and over the aquarium edge. There are a few options available to accomplish this task though.

The first one is by simply sticking some air line up the inlet pipe so it is as high as it will go and then sucking the air out until you effectively get a mouthful of water. When the pump is turned on any remaining air should be blown out of the outlet. If you choose this option then I would recommend putting a gang valve onto the air line so that you can have a break.

The second one is by drilling a hole in the very top area of the pipe where it goes over the aquarium edge. Into this hole insert some tubing and seal it in place using sealant. It is imperative that a very good seal is made as you do not want any air to get into the tube. Install a gang valve onto the airline and then suck out the air. Once you get a mouthful of water close the gang valve. When the pump is turned on the water will flow however this will create pressure on the gang valve therefore I would recommend that you bend this over into the aquarium and open the gang valve. As the pump is running a small amount of water will flow out of the airline tube into the aquarium.

That’s how closed loop plumbing effectively works however what are the benefits and are there any other considerations?

Before the considerations let’s have a brief look at the benefits of closed loop circulation.

The main benefit is the actual water movement which is created in the aquarium. Due to the large outlets which can be used and the powerful pumps which can be implemented the flow is very powerful however it is also very wide and soft. This is much more beneficial for corals as they require a lot of water movement but they do not want/like water movement which is forceful, in actual fact it can damage them sometimes even tearing the coral away from its skeleton.

Another benefit 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 above is the main benefits now let’s move onto some considerations.

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. Even better is to use rigid pipe which  is slightly flexible which will allow you to create a gentle curve around corners.

When the plumbing is complete and all the outlets are in place it is highly recommended that you perform a dry run. This is probably not the best name for it as water is involved but what I mean is that you fill the aquarium with household water and then test the loops. If you locate a leak then you can lower/drain the water and rectify the problem. If you fill the aquarium with reverse osmosis water, add salt, test and identify a problem then this is a waste of reverse osmosis water and salt – both of which are expensive.

All devices at some point will require maintenance or may even fail altogether. With a closed loop system the pipes are continuously full of water so if you remove the pump then you may get a wet floor as the water continues to flow through the inlet. I would recommend that ball valves are installed into both the inlet pipe and the outlet pipe. Should you need to remove the pump for whatever reason then you simply need to close the ball valves prior to removal. Once the pump is reinstalled the valves can be re-opened plus you won’t need to re-prime the loop!


Closed Loop Water Circulation
Rate this post
8 Comments
  1. Interesting article !

  2. Thanks!

  3. actually i want to separate 100 % air block in the closed loop system

  4. Hi

    Thanks for the providing interesting and useful article.I really like your blog, and i got so much information by your blog, I also bookmarked your page.

    Thanks

  5. Hello, glad you visited and found the site useful.

  6. nice to see your article. its really interesting. Thanks for such an informative blog.

  7. Thanks, visit again.

Leave a Reply