How to Prevent Aquarium Sump Overflows

Prevent Sump Overflows During a Power Outage

Aquarium in the dark
Aquarium in the dark. Markus U./Flickr/CC BY-SA 2.0

Saltwater (and freshwater) aquarium sumps are a great innovation and a terrifically useful piece of aquarium system equipment. Sumps do have one a drawback, however.

Many people don't understand that once their sump pump stops pumping and their tank water level has dropped to the overflow level, their tanks will continue to drain into their sumps via the pump outlet in the tank. If your pump nozzles are even 3" below the water surface, they can suck a lot of water from your tank into your sump.

So how can you avoid a sump overflow (and a very wet floor) in the event of a power outage? To start with, perform this test to determine if you have a potential problem:



Time Required

10 Minutes

What You Need

  • Bucket or another water container.
  • Water scoop.
  • Drill motor.
  • 1/8" drill bit.

Here's How

  1. Have a bucket and scoop (a coffee cup works fine) standing by.
  2. Top off your sump to your maximum "normal" level while the tank is running.
  3. Turn off all water pumps and skimmers in your tank.
  4. Watch the water level in your sump as it rises.
  5. If the water level starts to go above approximately 1" below your sump rim, start removing water into the bucket to keep it below that level.
  6. If the sump water level doesn't go above that point, you are fine.
  7. If you had to bail water, turn the pumps back on and mark the water level in your sump when everything is running normally. This is your "maximum fill" sump water level during normal operation.
  1. If you had to lower the sump water level during the test, there are a couple of simple solutions:
  2. Don't fill your sump above the "maximum fill" mark.
  3. Unplug the return pump in the sump and raise the pump nozzle above the water in the aquarium.
  4. Drill a small (1/8") hole in the side of the pump nozzles, just below your normal tank water level.
  1. Perform the above test again to make sure that the problem has been solved.
  2. Adjust the "maximum fill" mark on your sump.
  3. Once you are confident that your sump will not overflow, even during a power outage, you should sleep a bit better knowing that you won't wake up to a wet carpet in the morning.


  1. Read How To Keep Your Aquarium Running When the Power Goes Out and More Power Outage Tips to help you prepare for an emergency.
  2. Periodically check to make sure that the hole you drilled in the side of the nozzle is not clogged with debris. Inserting a toothpick in the hole will usually keep it open and working properly.

To help avoid overfilling your aquarium system when topping off, consider installing an automatic top off system. These systems not only help prevent accidentally putting too much water in the system when topping off, they also take the drudgery out of having to periodically haul water from your faucet to the tank and pouring it in. The DIY Automatic Tank or Sump Top Off project is easy to complete and the components are easy to find and relatively inexpensive to purchase.

Spending the few minutes that it takes to drill this one little hole is well worth the effort. Knowing that your sump will not overflow in the case of a power outage will help you sleep better at night.