Many reef tank and aquarium enthusiasts are well aware of how useful an overflow box can be. The main benefit of using it is that it ensures your display tank maintains the proper water levels at all times.
The overflow tank pumps in fresh water steadily to replenish evaporated water, as it can sense when your display tank water level gets too low. It also pumps your aquariums water through a filter so that impurities are removed.
My pick is the CPR CS90, continue reading for more options.
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Types of Overflow boxes
Overflow boxes can be internal and external.
Most of the time, the internal version is the better choice. You can customize it more easily, and it’s even more affordable than the external version. You also put this under the tank and you can cover it with a cabinet. But for these to work, the main tank needs a drilled hole.
So if your aquarium doesn’t have predrilled holes then the external version may be a better choice. It’s simply too bothersome to have to drill the hole in your aquarium yourself.
The external version may also be better if you don’t have enough space under your reef tank. This is usually a HOB (hang on back) unit. You can hang it at the back, and it’s not that obvious because you hide it with your corals and other tank décor.
This doesn’t need a drill because they use a siphon to suck the water over the side of the tank. Its main disadvantage is that in some rare cases, air can enter the siphon so the water stops draining to the tank. So when your return pump continues to put back water in your tank, eventually it will overflow.
That’s why you should check if the overflow can draw out any air that enters the siphon so it remains primed.
Finally, an overflow box is usually clear, so it’s easy to check how much water is inside behind the corals and the décor.
Buying a Hang-On Overflow Box
There are several ways to determine which HOB overflow box is right for your aquarium size. Usually, the manufacturer will help you out by simply stating what aquarium size range its overflow box is for.
You also need to make sure that the flow rate of the overflow box is rated higher than the return pump flow rate.
So if you have a 500 gph return pump, you can use a 600 gph. One simple guide you can follow is to get the size of your tank and multiply it by 10. So if you have a 90-gallon tank, that’s 900. Then get the next highest overflow rate, such as 1000 gph.
Strictly speaking, you can get a bigger overflow tank with a much higher flow rate, since the overflow will regulate itself and it won’t pump in more water than the amount that flows into it. But then a bigger overflow box will be more expensive, and you’re paying for greater capacity that you don’t really use.
Other factors to consider include the following:
- Is it easy to install?
- What are the chances of clogging?
- How’s the build quality?
- How noisy is it?
Since this will transport water from the tank, it’s up to you to get the sump, refugium, or wet/dry filters you need for your tank.
Top rated aquarium overflow box
1. Eshopps Overflow Box
In particular, we’re talking about the PF-800 overflow box, which Eshopps recommends for 75 to 125-gallon aquariums.
This box measures 8 by 3 by 10 inches, which means you only need 3 inches of space between the wall and the tank (better make that 3.5 inches to be sure).
This includes the pre-filter box, U-tube siphon, cylinder foams, and the necessary nylon screw and wing nuts.
Pros
- It pretty much works as it ought to, according to many customers.
- The installation is easy for most aquarium veterans.
- It is quieter than other models, although of course it’s not absolutely quiet.
Cons
- If you’re not an aquarium veteran, installing this is not as easy as it looks. There’s often no setup instruction manual included.
- Also, you may not know what kind of tweaks to perform to get it to work properly.
- Quite a number of people are skeptical that this actually has an 800 gph flow rate.
- It’s not easy to prime, and you may have overflow problems if there’s a power outage.
2. Aqueon Hang-On Overflow Box
This is also for a 125-gallon tank, although they also have a 400-gallon version.
It can be hard to do some research on this, because Aqueon doesn’t even offer a description of its features on its own website, apart from the fact that it measures 8.3" x 6.3" x 11.5".
In any case, its design does lead to an easy setup. What’s more, it uses a dual drain design. So it can still handle quite a bit of water even if one if the drains get clogged.
Pros
- It supposed to have a 2200 gph capacity, so it should handle return pump flow rates well.
- The setup is relatively easy, which should be true of any HOB option.
- It’s quite consistent in maintaining siphon.
Cons
- There have always been a sizable number of people who doubt Aqueon build quality. So you should check first for leaks over the sink before you hook it up. A handful of people has complained that their overflow box falls apart at the seams only after a couple of years.
3. KollerCraft TOM RP3 Overflow Box
This measures 8 by 7.5 by 6.5 inches (LHW) and can mount to tank frames of up to 1.75 inches thick. It’s also rated for 800 pumps.
Its gravity system primes automatically and it also comes with an anti-siphon feature to prevent flooding.
Pros
- It comes (or it should come) with the proper clamps and hoses. The installation and setup is easy.
- When everything goes right, it seems perfect.
Cons
- Quite a few people complain that they don’t get the hoses or the clamps that go with the purchase.
- Some people complain that it’s overrated in its flow rate capacity.
4. CPR CS90 Overflow Box
This model is rated for 125-gallon tanks but it has a 600 flow rate.
It offers an easy way to adjust the water level, and a black top to discourage algae growth.
Pros
- It’s very easy to clean and use.
- It deals with power outage effectively.
- It’s not that noisy.
Cons
- Just make sure that your return pump has a lower flow rate than 600 gph.
- You have to use it with the Aqua Lifter pump to maximize its efficiency.
Final recommendations
Each unit here does well—except when they don’t for some people.
So our pick is all about consistency, and you can get that with the CPR CS90 (as long as you also get the Aqua Lifter pump to be sure). This model garners the most consistent rave reviews, and you rarely—if ever—hear any complaints about failures or missing pieces.
Send in your questions, submit your recommendations, and give your opinions! What do you think is the best for your overflow box?
