Flood-proof Tank/Sump


Reefing newb
I'm finally putting together my first sump and in the process of putting the plumbing together. Only thing that worries me now is having either the main tank or the sump overflow on me during an outage. We tend to get them rather often for whatever reason around where I live, coincidentally we had one last night for six hours. I need to be 100% sure that if the power does go out that I don't get a pool in my room. I was told that I should drill a small hole in return pipes that would sit just below the water line in the main tank and that would break the siphon if the power ever did go out. I posted a pic of the overflow box I'm using. Maybe that can give someone an idea of whether it's a good one to use and if I have anything to worry about and what more I can do to prevent flooding during an outage.

My phone refuses to send the pic to my email so I'll post it when I can. Basically it has a big U-tube that transfers the water from the tank to the back into the overflow part and that has two different chambers, it's split half and half and one side fills up and pours into the drain side.
How to prevent the main tank from flooding:
  • Make sure the overflow is not overwhelmed by the return flow.
  • This means getting an overflow system that is rated higher than the return pump.
  • Also make sure your overflow system does not get clogged.
  • Better yet, have redundancies for your overflow - have dual (or more) overflows so if one fails, others are still functioning to allow you time to address the problem.

How to prevent the sump from flooding:

  • Drill proper siphon breaks at the return lines under the water level of the display tank.
  • Make sure the sump has enough volume to hold all the overflow water.

    1. Calculate how much water will drain from the display tank when the return pump is off.

    An example of this formula: Aquarium length X Aquarium depth X (h1 - h2)
h1 is the height of the water level in the main tank during normal operation.
h2 is the lowest point in the tank where water will overflow into the sump. This is usually the level of the oveflow box's lip.​
2. Calculate the normal water volume in your sump during normal operation.

3. The total capacity of your sump tank must be more than 1 + 2, taking into account displacement by other stuff you are putting inside the sump.​
Here is an easier way to figure out how much water can be in your sump and tank without having to calculate anything.
1)Turn off return pumps.
2)Fill display tank to where the water just starts running through the overflows
3)Fill the sump to an inch or two below the rim
4)Turn on return pumps and let the system run for a few hours to make sure any air pockets have worked their way out of the plumbing
5)Take a sharpie and mark the water level on the out side of the sump.

As long as you have this level or less of water in your sump, losing power will not cause your sump to overflow.
Couple questions regarding what was posted. One, why would I completely fill my sump up if it has different chambers (three in this case). That defeats the purpose of having three different areas in the sump doesn't it?

Second, where's the best place to drill holes in the return pipes below the display tank water line. About an inch below the line good or should it be higher/lower?

Here's my overflow box that I'm using.

the best place to have a hole is close to the highest point of the return line. if you have a reef ready tank it would be inside the overflow near the top of the pluming to where it goes into the tank. the reason to fill the sump almost all the way is so that it gives you room. get it almost full then turn the pump on. this way it will give u a running level. when u shut it off it should not go over the full mark.
Couple questions regarding what was posted. One, why would I completely fill my sump up if it has different chambers (three in this case). That defeats the purpose of having three different areas in the sump doesn't it?

View attachment 19708

This is to simulate the worst case "power down" scenario where the return pump is off and the water from the main tank drains as much as it can into the sump.
If you have a sump big enough, it is possible to have the display tank drain all the way down to the overflow level and not fill up the sump. This depends on the size of the display and sump tanks, as well as the level of the overflows and siphon breaks.

Your sump would not be filled with this much water during normal operations.
it depends on where the water outputs are at if they are down far in the tank it will drain until it gets air to break the siphon. that's why u need a hole drilled somewhere high. mine is in the elbow of the pluming where it enters the tank when the pump is on it has a little stream of water that shoots out but as soon as the pump is off it sucks air and breaks a siphon
just get a large enough sump to hold the water during a power outage. siphon breaks can be annoying and require maintenance that is easily forgotten to keep them clean to work. use a bean animal overflow design so your tank never floods, size your pipe accordingly (1-1.5") more than likely. and so you don't have to deal with all the water in your sump when the power goes out as that can create skimmer issues upon power being restored with high water level (unless you are using a soft start DC skimmer pump or apex to control a delay in start of your skimmer), use a georg fischer wye check valve. don't go cheap and get a flapper design as it will fail you much sooner and are basically impossible to service. good luck. there's tons of good info for you here.
I'm thinking the primary thing here is knowing how big my sump is. It's a 30 gallon long tank. I put a picture below to see it. My highest baffle wall is probably 4-5 inches from the top and that would be the skimmer area. I'm thinking if I did have an excess of flow down from the overflow, it wouldn't be enough to overfill the sump. If I drill holes just below the water line, it should be only a couple gallons at the most that would continue to flow down until the siphon broke and that would be it. How big should the hole be, just a pinhole or maybe 1/8", bigger? I'm guessing you want it big enough so it doesn't clog but not too big.
you also don't want a mess of water splashing all over the place and onto your reflectors or light fixtures or outside the tank. the thing is to have redundancies. if you're set on using the siphon break and not 100% about your sump containing all the water during a power outage without them, then utilize a check-valve as well.
Sounds like an idea to me. Question is, I have my return splitting at a T at the top and they stretch out to each side of the tank. Should I go for two check valves, one in each end, or will one in the back in the main return pipe suffice?
use one prior to the split. most commonly found just above/after your return pump, before a manifold, easy to service area, and be sure it's in proper positioning (more than likely vertical) for best operation.