How to Reduce the Risk of Flooding with Hang-on-Back Overflows

Hang-on-Back overflows are popular for non-drilled aquariums or those without built-in overflows

Hang-on-Back overflows are popular for non-drilled aquariums or those without built-in overflows

Marine aquarium hobbyists whose tanks didn’t come with a built-in overflow or can’t be drilled to accommodate one (i.e., those made of tempered glass) sometimes choose to use a siphon-based hang-on-back (HOB) overflow to feed water from the aquarium to a sump. These overflows come in different designs, but they’re all more or less a variation on the same theme.

In very simple terms, they consist of two chambers: a slotted box that is positioned inside the tank and skims water from the surface, and another box that hangs outside the tank. Water is drawn from the inside chamber to the external chamber via a siphon tube (or sometimes more than one tube). The water then flows under the influence of gravity out the bottom of the external chamber, through a hose, and down into the sump, where it’s pumped back to the main tank through a return hose.

While this type of overflow can be a viable alternative to built-ins, it does have some possible drawbacks, not the least of which is the potential for flooding your fishroom in the event of a power outage, pump failure, or loss of siphon. But if you do have an HOB overflow, there are some simple steps you can take to reduce this risk so you can rest easier whenever you’re out of the house, asleep, or otherwise unable to access the aquarium immediately. They include:

Choose the right sized sump and pump

In the event of a power or pump failure, water will continue to flow down to the sump until the level in the main tank drops below the slots in the internal chamber. If your sump can’t accommodate all the water to that point, it will overflow onto your floor. If you have any doubt, get yourself a bigger sump!

Of course, you’ll also need to make sure the return pump is properly rated for the overflow system you’ve purchased so it doesn’t pump water back to the display tank faster or slower than the overflow can deliver it to the sump. Your dealer can advise you which pump is appropriate for your overflow.

Drill the return

Another potential drawback to HOB overflows is that if power is lost or the return pump fails, the return hose (through which water is pumped from the sump back to the display tank) can act as a reverse siphon, pulling water from the display tank down to the sump until the mouth of the nozzle finally catches air and breaks the siphon. Again, depending on the size of your sump, how deep in the tank the nozzle extends, and the size of your aquarium, this may or may not result in flooding.

To eliminate this problem, all you have to do is drill a small hole in the nozzle just below or right at the water line so it draws very little water down to the sump before catching air. Nowadays, many of these systems come with these holes pre-drilled, so you don’t have to do it yourself. In any case, be sure to keep this hole clean of algae, calcium deposits, salt creep, etc. so it does the critical job it’s supposed to do in an emergency.

Guard against loss of siphon

Flooding can also result if the siphon that transports water from the internal overflow box to the external one gets disrupted. If this occurs, water will cease flowing down to the sump but the return pump will keep right on pumping, potentially overflowing the display tank and exposing the return pump to air. To prevent this:

  • Make sure the internal and external overflow boxes (especially the slots in the internal one), siphon tube, return pump and hose, etc. are kept clean and well maintained. This will ensure vigorous water flow through the overflow system, thus preventing air bubbles from getting trapped in the siphon.
  • Verify that in the case of a power outage, both openings of the siphon tube remain submerged in water after the flow has stopped completely. This will ensure that the siphon doesn’t break and flow will resume automatically when power is restored. Most HOB overflows these days are designed such, but it’s important to confirm this nonetheless.
  • Watch for bubble buildup! The siphon tubes in HOB overflows have the tendency to accumulate air bubbles at their highest point. A few little bubbles won’t make much difference, but over time, a few little bubbles can become one big bubble and, ultimately, an air pocket that breaks the siphon.
  • If you see bubbles starting to build up, raising the tube and lowering it with a bit of force several times (without removing the open ends from the water, of course) will often clear them. But again, this shouldn’t be a significant problem if the flow rate through the tube is sufficiently robust.

Also note that some HOB overflow systems are designed to be used with a lift pump that prevents air bubbles from accumulating in the siphon tube and quickly restores flow when power resumes after an outage.

Another step worth considering is using a float switch in conjunction with your return pump. Setting the float switch so the pump will shut off automatically if the water level in the sump dips too low will not only prevent your display tank from overflowing in the event of siphon loss, but it will also protect your costly return pump from running dry and burning out.

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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.

Comments

  1. Paul Baldassano says:

    You can also buy, (or make) a water detector under your tank in case there is water on the floor. Sometimes you can have a leak go un-noticed for a long time especially if, like me, your tank is built into a wall. I built a leak detector from a GFCI that shuts off my skimmer pumps if the bucket overflows or there is a leak. What I made is probably illegal (although it is safe) so I won’t give instructions. I don’t want the GFCI police to come and break down my door.

    • Jeff Kurtz says:

      I’ve heard those GFCI police can be as bad as the IRS–or is it the NSA? One of those acronyms.

    • Joe Casey says:

      I do not understand, “Verify that in the case of a power outage, both openings of the siphon tube remain submerged in water after the flow has stopped completely. This will ensure that the siphon doesn’t break and flow will resume automatically when power is restored. Most HOB overflows these days are designed such, but it’s important to confirm this nonetheless.” Specifically “This will ensure that the siphon doesn’t break and flow will resume automatically when power is restored. If the siphon does not break, wont it continue to drain the tank? I also did not understand “both openings”

  2. First of all always fill you system with the power off never with the power on this will make sure that your system is in balance.
    Also you can drill a whole in the siphon tube and use a power head to suck the air out with a hose connector and hose. There are over flow systems on the market ready to use per tank size.
    Always test your system and mark with black marker where a water level problem may be.

  3. “Of course, you’ll also need to make sure the return pump is properly rated for the overflow system you’ve purchased so it doesn’t pump water back to the display tank faster or slower than the overflow can deliver it to the sump.”

    Surely it doesn’t matter if your return pump is pumping water back into the aquarium at a slower rate than the overflow drains…?! Water will only drain if more water is being added.

    • Slower is fine,… but if the return pump returns water faster than the system brings it to the sump then you can:
      1) cause the return pump to run dry which could seriously damage it
      2) if the sump contains more water than the difference in height between your overflow and the top of the main tank, you can overflow the main tank.

      • My point is that he said that it isn’t OK for it to be slower. I get that faster is obviously is a problem.

        • You’re correct, Billy. The return pump can (and should!) be slower than your overflow siphon. Anything else requires you to meticulously balance your pump to your overflow, which is for all practical purposes, impossible. Been there, done that.

          I wish this article would have mentioned it, but the best rule of thumb is to have an overflow that is rated for higher flow than your pump can ever possibly hope to pump into the main tank on its best day. This way, no matter how much your return pump feeds the tank, the same amount will drain back into the sump, keeping it in balance.

          Can’t understand why the article didn’t explain that… :\

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