What Is An Aquarium Overflow Box And Who Should Use Them?

What are the best aquarium overflow boxes?

Whether you have a freshwater or reef tank, investing in the right equipment will make your aquarium better to look at and more practical to clean.

An aquarium overflow box is one way to keep your tank cleaner.

But what is it exactly, and why should you invest in one?

Is it for everyone? And if so, what is the best overflow box?

Aquarium Overflow Box

Eshopps PF300 Overflow Box

  • Good for small tanks
  • Easy to install
  • Fits into small spaces
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Eshopps Overflow Boxes

  • Many size options from 75 gal to 500 gal
  • Good looking option
  • Quality components
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Prodigy External Overflow Box

  • Stylish external box
  • Slimmest overflow at only 8mm
  • Reliable overflow box
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If you plan to set up a sump, an overflow box will be an excellent entry point.

It allows you to transfer the aquarium water to the sump underneath the cabinet.

At the same time, you can easily install wet and dry filtration systems to keep your tank in excellent condition.

The type that you need will depend on a variety of factors from:

  • The type of fish you are keeping
  • The size of your tank
  • What other filtration and tank care devices you already have set up

For example, if you are already using a dosing pump, that might affect which overflow you get.

Make sure you read the reviews of those who have tried and tested these overflow boxes in order to get the full picture of capabilities and best use cases.

1. CPR CS90 Fish Tank Overflow

diy aquarium overflow

This overflow box features one of the most efficient draining mechanisms, helping to maintain your tank in excellent condition.

It’s suitable for glass and acrylic aquariums up to 125 gallons and features an adjustable water level to help you change your tank’s water level.

Your fish will enjoy a bigger room for swimming, and you won’t struggle because the overflow box is extremely quiet.

It comes with failure prevention options, guaranteeing that your overflow box will last for long.

It has a big prefilter that prevents clogs by snails. However, you need to buy the aqua filter pump separately, which will add to the overall cost of this overflow box.


  • Efficient draining system
  • Suitable for glass and acrylic tanks
  • Adjustable water level
  • Failure prevention options
  • Prefilter to prevent clogs


  • You need to buy a pump for the aqua filter separately

2. Over Flow Box For Large Aquariums

best external overflow box

This overflow box is suitable for medium-sized aquariums that have a capacity of more than 150 gallons.

It comes with all the needed tools and pieces, so you can easily install it in your tank.

Whether you have a glass or acrylic aquarium, this overflow box will be an excellent choice because it’s easy to install.

It allows you to adjust your tank according to your needs, so you can easily install a refugium, a sump, or wet and dry filtration systems.

It transfers the water from the top of the aquarium to the external filter without making any expensive modifications that can mess up your tank.

Thanks to its minimalist design, you will have more swim room for your fish and plants to create a tank that matches your taste.

You might need help to drill the tank for installation. However, once you’re done, the overflow box is plug and play, allowing you to manage your tank successfully.


  • Suitable for medium-sized tanks
  • Works on glass and acrylic tanks
  • Minimalist design
  • Leaves room for fish and plants


  • You’ll need to drill the tank to install this overflow box

3. Eshopps Overflow Box

hang on back overflow

This easy-to-install overflow box is the perfect choice for a small tank.

It’s designed to fit right between the wall and the tank to maximize space for your fish and aquatic plants.

If you have a small space available for your tank setup, this overflow box will be an excellent option because you only need 3 inches to install the overflow box.

It already comes with everything needed for fast and straightforward installation. It comes with a foam prefilter, so it filters the aquarium promptly.

Thanks to the gravity-feed system, there will be no need for complicated installation or drilling.

However, compared to other models, this one can be a little bit noisier.

Some modifications are needed to install it properly for silent operation.


  • Suitable for smaller tanks
  • Designed to fit into tight spaces
  • Easy and straightforward to install
  • No drilling is needed


  • Quite noisy compared to other models

Types of Overflow Boxes

There are several types of aquarium overflow boxes, and some of them don’t require drilling the glass.

This is why you need to be careful about the type and size of the overflow box before buying one to install it in your tank.

1. Internal hang-on overflow boxes

These are the most straightforward models to install, so they will work better if you’re a beginner or you don’t have a big aquarium that you’re trying to manage.

However, these models are most likely to fail. They usually fail when the siphon stops working because of a power outage or the failure of pumps and tubes.

If you’re on a tight budget, this overflow box will work for you because you can install it without drilling the glass. You can also overcome its failure points by building a larger sump.

2. Internal built-in overflow box

If you’re looking for something more advanced, you need to invest in an internal built-in overflow box.

This is the overflow box that you’re likely to see on reef-ready systems.

The overflow box is integrated into the body of the tank and is more expensive.

3. External overflow box

Serious aquarists and those who are willing to make the biggest investments should invest in an external overflow box.

There are currently compact designs that have a small footprint inside your tank, and would still perform exceptionally to keep your aquarium in perfect condition.

There are 2 disadvantages associated with this type of overflow box:

  • The first one is that you can’t install your aquarium to the wall because your overflow box will stick out
  • The second one is that you will have to drill the glass or hire someone to drill it before you’re able to install the overflow box

For these reasons, many choose one of the other two styles.

Fish Tank Overflow Box Size

Overflow boxes are self-regulating, and this means that they won’t drain more water than the amount that flows into them.

This is why you need to make sure that the overflow box you’re picking is suitable for the return pipe you’re buying.

You can start by shopping for the return pump and then buy an overflow box with a flow rating higher than that of the pump.

If you’ve already picked a box, make sure that the return pump you pick is rated lower than the flow rate of the box.

You can also calculate the adequate size by taking the size of the aquarium and multiplying it by 10. After that, you can round the number to the nearest number that exceeds the result.

For example, if you have a 55-gallon tank, you multiply this number by 10 and get an overflow box that is rated higher than 550gph.

DIY Overflow Boxes

That’s part one of a tutorial on how to make a DIY overflow box. You can check out part 2 here.

It will take a bit of time and money, but it’s not all that difficult to make your own overflow.

If you just want to buy a new one pre-made, we chose the CPR CS90 Overflow Box as our top option because it’s suitable for tanks with a capacity of up to 125 gallons.

It’s quite easy to set up and comes with failure prevention options to guarantee that your overflow box will work for a long time.

There are other cool models on the list, so you can easily find a suitable one.

Make sure that you’ve picked the right size and type to enjoy a flawless aquarium setup.

Check out these top aquarium sterilizers and dosing pumps to get your water quality up there even further.

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