How an Aquarium overflow works and how to choose the right overflow


Overflows are great.  They allow you to hide all your ugly equipment outside of your display tank, allow you to use bigger, better, and more equipment, and they make for a steady water level in your display tank. One of the most common questions I see is “How big of an overflow do I need to buy” and it’s a bit ironic because the answer is so much simpler than most folks expect it to be.  I think the hesitation and the question comes in from the fact that how an overflow system works seems complicated though, like most things, once explained it turns out to be pretty simple.  If you’re looking for the simple answer to “What size overflow do I need?” The answer is:  Purchase an overflow that is rated for more flow (generally rated in Gallons per Hour) then your return pump will be. That’s it.  If you have a return pump that’s going to be 500gph, get an overflow rated for at least 500gph, as long as the overflow is big enough it will self-regulate itself to any pump slower.  In the case of an internal overflow, it’s worth mentioning that you want to make sure you have enough room for any fittings you plan to install as well, but generally speaking they are all sized to house typical size plumbing fittings anyway.    If you’re more interested in how the concept works, then keep reading!

Your typical self-installed overflow will fall into one of two categories. First is the internal overflow and second is the external overflow.  For the purpose of explaining overflows I am going to use the internal overflow as the example.  The principles with an external overflow are the same, just the means of getting water over the side of the tank is different and I will get into more detail with towards the end.

One of the biggest benefits (and most misunderstood concepts) of an overflow is how they maintain steady water levels in the display tank and are self-regulating.  Really this concept is just gravity at work.  When you setup an overflow in a tank you are essentially creating a dam.  The top of the dam is the overflow teeth.  You have the high water side of the dam (inside the tank) and the low water side (inside the overflow where the drain is).  The only way water goes over and into the overflow is if the water level in the main tank exceeds the height of the overflow.  The excess water flows over and then proceeds down the drain and into the sump.  This way the height of the water in the tank will not exceed the height of the overflow and when the power goes off the tank will only drain until it reaches the top of the overflow (though be careful about back siphons down your return pump line).

You might be asking yourself why don’t I skip the overflow and just put a drain pipe in.  Well the short answer is that you could.  That said though, the overflow offers a bit more flexibility and protection.  The overflow box and its teeth disperse the flow to a much wider area.  Say your return pump is pumping 500gph, so therefore 500gph is going over the teeth and into your 1” drain. At your 1” drain you have a 1” area that has 500gph flowing through it.  If you have an overflow, that’s still going to be the case except your aquarium critters aren’t exposed to the very small/high flow area, its being spread out across all the teeth on the 3 sides of the overflow so each tooth has a significantly less flow.  This helps to keep algae and your critters out.  They are also a lot more attractive than a simple pipe and in the event you need to replace or change a plumbing fitting, you only need to drain the overflow box instead of your entire tank.

Every tank has evaporation and in a tank without an overflow, sump, etc that happens right in the aquarium and is noticeable in the display. Another benefit of using an overflow and sump is that it relocates the signs of this evaporation to the sump and in particular the last chamber (assuming you have chambers) that holds the return pump.  The water will remain steady in every other area.  The reason for this is that the only amount of water that exceeds the height of the overflow box will spill into the overflow and back down to the sump.

This can be difficult for some people to make sense of so visualize this:  You have a 10g tank filled up to the overflow and a sump with 5 gallons of water in it.  Your move 3 gallons of water from the sump up to the display, how much goes over the overflow and back down to the sump?  3 Gallons of course.  Let’s say in the process of doing this 1 gallon of water evaporates from the display tank.  You add your 3 gallons of water to the tank that holds 10g, but currently has 9g in it, how much overflows into the tank?  How much water is left in the sump?  Of the 3 gallons 1 is replacing the water that evaporated so only 2 gallons drain back down to the sump.  Of the 5 gallons the sump had before, 3 went to the display and 2 came back, which leaves you with 4 gallons in the sump.

Side Bar: The constant water level and self-regulating nature are the same reason folks put a baffle in there sump.  The baffle keeps a constant water level in the area before it and if water exceeds that level it spills over into the next chamber.  A constant water level is very important to the performance to a lot of equipment such as protein skimmers.  The level of water in the chamber effects how much flow the skimmer pump produces.  Higher water levels increase flow and vice versa, and it often doesn’t take much change to throw the skimmer out of whack.  A baffle in the sump is basically the exact same thing as the overflow is in your tank. 

When it comes to overflows they generally come in two different types, Internal & External.  Each has their upsides and their downsides. Generally speaking most folks are going to want an internal overflow if at all possible. The advantages and disadvantages of each are:

Internal Overflows:

interal aquarium overflow interal aquarium overflow - side view

The construction of an internal overflow is very simple, they are basically a 4 or 5 sided box. The 4 sided boxes are open on the top and the back.   The back is secured with silicone directly to the back of the tank surrounding the drain hole.  The 5 sided overflow boxes are identical except that there is a back on the overflow box itself.  With the 5 sided boxes you do not need to secure the box to the tank with silicone; however you need to add an additional gasket on the bulkhead between the overflow box and the glass to prevent water from seeping between the box and glass and then out the bulkhead hole.  This isn’t needed on the 4 sided boxes because the bulkhead and single gasket would be directly against the glass.   An internal overflow box is more desirable then an external in almost every way.  They are less expensive, offer flexibility for much more customized or complex plumbing, etc.  The only catch is that it requires a hole in the glass.  Many tanks come with holes pre-drilled and if yours is not, it’s not that difficult to add one to most tanks but it does require the tank to be empty so you can lay it on its side. If that isn’t possible, then an external overflow box is the alternative.

External Overflow:

external aquarium overflow - side view The benefit to an external overflow is that you don’t have to drill holes in your tank.  Instead they suck water over the side of the tank by means of a siphon.  The unique design of the overflow allows them to maintain a siphon in the event of a power outage by “trapping” water in the bend over the tank so that this area is always full of water and only the excess water that flows into the overflow will drain down to the sump.  They do their job and are quiet effective.   It is generally advisable to only use an external overflow when it’s not possible to drill a tank because, while its rare, in the event the siphon in the overflow should break (air enters the siphon) water would stop draining to the tank.  As your return pump continues to pump water up to the tank and it has nowhere to drain, eventually it will overflow.  Modern hang on back overflows like the CPR aquatics overflows include a small pump that draws out any small amounts of air that may enter the siphon and help keep it primed.  While I personally haven’t ever seen one fail, nor talked to a person that had one fail on them, it’s still a best practice to use the internal overflow when possible just in case.

Choosing the right size:

This process is a lot more straightforward.    When it comes to sizing an overflow, you just need one that is large enough to handle the capacity of the return pump you plan to get.  Because the overflow is self-regulating, it won’t drain more than flows into it, if you have say a 500gph return pump; just get an overflow that is rated larger than 500gph.  It’s that simple.  Don’t have your return pump yet?  Fine, get an overflow and make sure that your return pump is smaller than the overflow. As a rule of thumb I usually take my tanks size, multiply by 10 and then round up to the next largest overflow.  So in the case of a 50g tank, 50x10=500gph.  The CS90 overflow from CPR is rated for 600gph and is the closest overflow in size that exceeds 500gph.   Technically though, there is no reason one couldn’t use the CS150 rated at 1,600gph on the same tank.  It just isn’t that practical as it is a more expensive overflow, uses bigger more expensive plumbing parts, and takes up more space on the tank.