Perhaps you’ve been keeping fish for a while.

You’ve got some nice hang-on filtration. You’ve upgraded your lights to support coral, which is now starting to grow. Or, perhaps you’ve started keeping messy predator fish. Your water quality was fine for the fish only system, but you need to step up your game if you want to keep expanding in the hobby.

You’re going to need to set up an external filter, like a sump or wet/dry system so that you can keep up with the aquarium’s needs. But, you don’t have a tank with an overflow.

You could go out and buy a whole new tank that is pre-drilled for an overflow, which is a huge ordeal for an established aquarium. You could try to drill it yourself, which could easily result in shattered glass, a huge headache, an upset spouse and the expensive new tank you were trying to avoid buying.

Or, you could take the next step without all that hassle by picking up an overflow box. While pre-drilled/reef ready aquariums work really well, they can be expensive and require a lot of work setting up. An overflow box is a great solution to increasing filtration while keeping the cost and hassle low.

An overflow box is a device that sits on the lip of your aquarium. As water is siphoned from the aquarium, it is delivered by gravity to the sump underneath, where it is filtered. Then, a large return pump in the sump delivers water back to the aquarium. As the water level rises in the aquarium, it cascades back into the overflow box to complete the cycle.

There are great advantages to having an overflow box. Have you ever noticed the shiny protein slick that looks like an oil spill forming on the surface of your aquarium? Organic molecules are attracted to the surface of the water (I’ll skip the lengthy chemistry lesson on why). An over flow box skims the surface of the aquarium, sending the dirtiest water to the external filter. Surface skimming also really helps oxygenate the water.

Now that your interest is piqued, let’s talk about how to go about setting up an overflow box. First, always be sure to read the manual carefully. Each brand of overflow box is different, but these steps and tips will set you up for success. We will be focusing on tips for setting up the overflow itself. But, we also have great articles on fully setting up a sumpfor the first time.

Choosing an Overflow box

Choose an overflow that will closely match the flow you want in your aquarium. A freshwater aquarium should turnover about 4x per hour. While a saltwater aquarium needs more flow, not all of that needs to be produced by the return pump. 10x per hour is a reasonable target. Just make sure that your return pump isn’t more powerful than your overflow box can handle. Your biggest goal in properly setting up an overflow box is to prevent any water from ever getting onto the floor.

Other Parts
You will probably need a few parts in addition to the overflow box. First, you will need a drain hose to deliver water from the overflow to the sump. Most overflows have 1” bulkheads, which means you will need a hose with a 1” PVC fitting (schedule 40). You can also buy a length of flex PVC pipe. You will need PVC cement to affix the drain hose to the overflow.

Most overflow boxes list an Aqualifter or venturi pump (like a Maxi-Jet) as an optional accessory. This is optional in name only. You want one unless you want a flood. Most hobbyists go with the Aqualifter, and many keep a spare on hand. Pick up some standard airline tubing to go along with it.

Setting up the overflow

Unpackage the overflow box, read the instructions, and make sure everything is there before beginning. Then, rinse the overflow box and other plastic parts to remove any residue leftover from manufacturing. Attach the bulkhead(s), making sure the O-ring is on the inside half. Then, attach the pre-filter screens to the inside of the bulkhead. Then connect the drain hose. If possible, wait to glue the drain hose until you are certain everything is working properly.

Next, set the overflow on the lip of the aquarium. Adjust the height of the overflow so that it is just below where you want the surface of the water to be. Adjust the tilt of the overflow (if possible) so that it is level with the tank water.

Before starting the overflow, fill the sump to the operating level. It is a good idea to have extra water prepared just in case you need more. Both the inner and first outer part of the overflow need to be filled with tank water. Just grab a cup and fill them.

Your next step will be priming the overflow. The area in the middle of the overflow will be filled with air. You need to prime the overflow by siphoning all of the air out. Here is where the Aqualifter comes in. Attach some airline tubing so that the Aqualifter is pulling the air out of the overflow box and sending it back to the tank. Once all of the air has been pulled out, water will start to flow through the overflow and down to the sump.

Leave the Aqualifter attached and running at all times. Over time, bubbles can build up inside the siphon tube. When that happens, siphon can be lost. Siphon can also be lost during a power outage. The Aqualifter ensures that siphon will be regained if lost. Losing siphon would be bad because the return pump would still be operating. All of the water in the sump would be pumped out, possibly spilling onto the floor. This is why an Aqualifter is not an optional accessory.

As soon as the overflow is working, turn on the return pump. The cycle will then be complete. Add water as necessary to get the water level in the sump where you want it. Check all around for leaks. Make adjustments to the height of the overflow box.

Now that everything looks good, it is time to simulate a power outage. Turn off the return pump. See how the tank settles. Make sure that there is not too much water in the sump. During a power outage, the sump should not overflow. Turn the return pump back on to ensure the overflow starts back up.

Sometimes, an overflow will ‘burp.’ Air will build up in the drain line and will be expelled periodically, followed by a flushing sound. This can be quite annoying. You want as much water as possible flowing through the drain pipe to reduce the amount of air buildup.

This is why your return pump should be pushing almost as much water as your overflow box can handle. You do not want to limit the water flow going down the drain pipe. But, if you put a ball valve on the line coming out of your return pump, you can adjust the flow. This can make a big difference. Also, many overflow boxes have adjustable pre-filters. Changing the height of the pre-filter can solve the problem. Finally, do not put the end of the drain hose under water, which will promote the ‘burping.’

Hopefully, you found this information on setting up an overflow box helpful. As always, if you have any questions our techs are available to help you succeed in the hobby. We would love to hear from you!