A Beginner’s Guide to Hang-On-Back Filters for Marine Aquariums - Getting Started

Welcome to Simple Reefs. We are a brand new website designed to take the stress and expense out of starting your very own marine aquarium. Today, we are giving you A Beginner’s Guide to Hang-On-Back Filters for Marine Aquariums.

This article explores what a Hang-On-Back filter is, how it can be used for a freshwater to marine aquarium conversion, a Hang-On-Back filter’s pros and cons, and we are also going to take a look at when a Hang-On-Back filter filter isn’t a good idea.

Venturing into the vibrant world of marine aquariums and reefs can seem daunting, especially when all you see are complex aquarium setups with elaborate sumps and copious amounts of plumbing. If you are coming straight from the world of freshwater aquariums, it can all be a little bit overwhelming.

When you combine this with the all too common rhetoric regarding sumps and how they are an essential part of marine aquarium life. It’s no surprise than many potential reef owners give up before they have even tried. The honest truth is, that needn’t be the case and for one simple reason.

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A Beginner’s Guide to Hang-On-Back Filters

Something that a lot of people in the marine aquarium hobby simply don’t like to admit is that sumps are not essential for starting a thriving marine aquarium. You can take an affordable sumpless tank or even an old freshwater aquarium that you are no longer using and easily turn them into stunning marine setups.

In fact, we talk all about the different options available to you should you want to do just that in this article right here. One of the best, most affordable, and easiest ways to do that is with a Hang-On-Back filter. For those of us with smaller tanks or those looking for a simpler entry point, the humble Hang-On-Back (HOB) filter, a staple of the freshwater hobby, can easily be adapted for a saltwater environment.

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What is a Hang-On-Back Filter and How Does It Work?

A Hang-On-Back filter, often referred to, simply, as a HOB, is one of the most popular types of aquarium filtration systems. Especially for small to medium-sized tanks. It is an external filter, meaning its main body sits outside the aquarium, typically hanging on the back rim of the tank, which saves valuable space inside. It also reduces unsightly equipment ruining your display.

A Hang-On-Back filter typically consists of a few key components:

• An intake tube that sits inside the aquarium to draw water in.
• A motor that powers an impeller (a small, spinning rotor).
• A filter box or chamber that holds the filter media.
• An outflow spout or spillway where the filtered water returns to the tank.

As you can see, it’s a pretty simple idea. Pull water in through a bunch of filter media that removes dirt and impurities from the water and spit it back out. That leads us nicely onto our next question.

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How Does a Hang-On-Back Filter Work?

The beauty of a Hang-On-Back filter is in its ease of use and simplicity. The mechanism is really basic yet incredibly effective:

1. Water Intake: The motor creates suction and pulls aquarium water up through the intake tube. A strainer at the bottom of the tube prevents fish and large debris from being drawn in. Sometimes this will come in the form of a piece of foam.
2. Media Filtration: The water enters the filter box and is directed through various stages of filter media. In a typical setup, it first passes through a mechanical filter (like a sponge or floss) to trap larger pieces of physical debris. It then flows through chemical media (like activated carbon) to remove impurities, smells, and discolouration, and finally through biological media where beneficial bacteria breaks down waste.
3. Water Returns to Tank: Once the water has passed through all the media and is clean, it fills the chamber and overflows, cascading back into the aquarium via the outflow spout or spillway. This waterfall effect is highly beneficial as it breaks the water’s surface which creates surface agitation. This surface agitation promotes crucial gas exchange by releasing carbon dioxide (CO2​) and dissolving oxygen from the air into the water.

Hang-On-Back filters come in many different shapes and sizes tailored to a variety of different Aquarium setups. Prices range from the ultra affordable to the slightly more expensive but never venture into the same lofty territory as canister filters.

If you are looking to turn an old freshwater aquarium into a marine aquarium or simply want to purchase a cheaper, sumpless, aquarium. A Hang-On-Back filter is the ideal solution. With that being said, what should you look for when purchasing a Hang-On-Back filter?

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What to Look For When Purchasing a HOB Filter for a Marine Aquarium

Not all Hang-On-Back filters are created equal, especially for the demands of a saltwater tank. When choosing a model, prioritize flexibility, size, and power over additional proprietary, and often useless, features. We want to make sure we have ample space for a decent range of filter media and plenty of power to turn the water over many times per hour.

Here are the key factors to consider:

• Large, Customisable Media Basket: This is probably the most critical feature to look out for with your Hang-On-Back filter. Avoid models that force you to use specific, pre-made cartridges. These are designed to make money by forcing you to stick with the company’s own products. They are often of an inferior quality, as well. Look for filters with a large, open chamber or “media basket.” This gives you the freedom to use your own media bags filled with high-quality carbon, GFO, filter floss, or other specialized media tailored to your own tank’s needs.
• High Flow Rate: A Hang-On-Back filter with a powerful motor is essential. The filter’s flow rate is measured in Gallons Per Hour (GPH) or Litres Per Hour (LPH). For a marine tank, you should choose a filter with a GPH or LPH rating that is at least 5-10 times your tank’s volume. For example, if you have a 200 litre tank, you want a minimum LPH rate of 1,000 Litres. It is always better to oversize the filter and have more flow than you need. Remember, the more water flowing through the filter, the more often it can be processed, the cleaner it will be, and the more surface agitation to further oxygenate your water.
• Adjustable Flow Control: While a high maximum flow rate is important, the ability to adjust and dial it in your Hang-On-Back filter’s flow rate is still a significant advantage. An adjustable flow allows you to lower the flow during feeding times to prevent food from being blasted around the tank or sucked into the filter, or to provide a gentler flow for certain types of corals.
• Reliability and Build Quality: A filter is the heart of your aquarium’s life support system. A failure can be absolutely catastrophic. Stick to reputable brands known for durable motors and quality construction to ensure longevity and protect your livestock. Reading user reviews can provide insight into the long-term reliability of a specific model and we will be reviewing a bunch in the near future.
• Self-Cleaning Impeller Technology: Some premium Hang-On-Back filters feature an impeller that briefly spins in reverse each time it starts up. This deceptively simple action is incredibly effective at dislodging any sand, small pest snails, or coralline algae build up that could otherwise jam the motor. This is a highly valuable feature for a marine tank, greatly improving reliability, reducing maintenance, and ensuring the filter restarts properly after feeding or a power outage.
• Surface Skimming Intake: Another premium feature found on some Hang-On-Back filters is a built-in surface skimmer. This attachment actively pulls water directly from the surface, removing the oily biofilm that often accumulates in marine tanks. This improves gas exchange and allows for better light penetration, which is crucial for coral.

Naturally, you may already own a Hang-On-Back filter from having a freshwater aquarium. If you do, there is no good reason why you can’t repurpose that for your marine aquarium conversion. Just make sure the principles outlined above are followed and make sure you follow our guide to cleaning and preparing your Hang-On-Back filter.

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The Pros and Cons of HOB Filters in Marine Aquariums

Naturally, everything when it comes to the marine aquarium hobby comes along with its own list of pros and cons. In my opinion, the pros with Hang-On-Back filters far outweigh the cons but let’s take a look.

Pros:

• Cost-Effective: HOB filters are significantly cheaper than drilling a tank and setting up a full sump system. They are even cheaper than a decent canister filter.
• Easy Installation and Maintenance: They are plug-and-play, require no plumbing, and both media changes and maintenance are quick and straightforward.
• Space Saving: Ideal for smaller apartments or rooms where a large cabinet with a sump is not feasible. The only space required is a few inches behind the aquarium.
• Excellent Surface Agitation: The return flow from a Hang-On-Back filter creates a waterfall effect that effectively breaks the water’s surface, promoting essential gas exchange.
• Versatility: The open chamber design of many Hang-On-Back filters allows you to customise the media you use, turning it into a chemical filtration powerhouse. They can even be turned into mini-refugiums with a little extra effort.

Cons:

• Limited Capacity: Hang-On-Back filters have a small volume compared to sumps. This limits the amount of media you can use and the overall filtration capacity. Bigger is always better when purchasing a Hang-On-Back filter.
• Potential “Nitrate Factories”: This is the single biggest risk but it is easily mitigated with frequent maintenance. If mechanical filter media like sponges or filter floss are not cleaned or replaced very frequently (i.e., every few days), the trapped organic waste will break down and produce nitrates (NO3−​), which can fuel algae growth and stress corals.
• Aesthetics: The filter unit is always visible on the back of the aquarium, which some hobbyists find less appealing than the clean look of a tank with a sump. This is one of the obvious sacrifices that comes from having a converted freshwater aquarium.
• Limited Equipment Space: Unlike a sump, a Hang-On-Back filter does not provide a place to hide heaters, protein skimmers, or dosing lines. However, Hang-On-Back protein skimmers are available as a separate piece of equipment.

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Limitations: When a HOB Isn’t an Acceptable Alternative

While excellent for small and medium sized setups, Hang-On-Back filters do have some clear limitations. In the following scenarios, a system with a sump is probably a significantly better, and often necessary, choice:

• Large Aquariums: For tanks larger than approximately 300 litres, a Hang-On-Back filter may lack the capacity to process the water volume effectively. This will mean you have to double up on your filters and this is where things start to get a bit inconvenient. Larger tanks may also have a greater bioload, meaning you will probably need other equipment like protein skimmers, large refugiums, and media reactors that will simply take up too much space in Hang-On-Back form.
• Demanding Livestock (SPS Coral Reefs): If your goal is to keep sensitive and demanding corals, such as Small Polyp Stony (SPS) corals, a HOB filter may not be adequate. These corals require exceptionally stable water parameters and pristine water quality. While I did successfully keep some of the more simple SPS (Montipora) with a Hang-On-Back filter, I am not sure you would have much success with thing like Acropora. A sump system is almost essential for housing the necessary equipment (calcium reactors, high-performance protein skimmers, dosing pumps) needed to maintain this stability.
• Need for Advanced Equipment: The desire to use multiple pieces of advanced equipment makes a sump more of a necessity. There are few practical ways to run things like calcium reactors, a large in-sump protein skimmer, a UV steriliser, an automatic top-off (ATO) system, and various other media reactors and refugiums on a tank with only a Hang-On-Back filter. A sump provides a centralised, hidden space for all this gear to operate efficiently.
• Aesthetic Goals and System Stability: For hobbyists seeking a clean, “rimless” look with no visible in-tank equipment, a sump or All-In-One tank is the only solution. By using an overflow box, all plumbing, heaters, and probes can be hidden below the tank. Furthermore, a sump adds significant water volume to the entire system, making it more stable and less prone to rapid fluctuations in temperature and chemistry.

There are some definite limitations to what you can do. My 260 litre tank was at the upper end of what would be acceptable with a single Hang-On-Back filter.

My equipment included the Hang-On-Back filter, a Hang-On-Back protein skimmer, an ATO, and a decently sized algae reactor growing Chaeto. By this point, I was seriously running out of room and my aquarium looked like some kind of fiendish science experiment with all of the tubes going into and out of it and all the equipment hanging off the back.

While I had fantastic water quality and thriving coral, I was pushing the limit of what was possible. If you want a large tank or a very clean look, All in One or an aquarium with a sump is the preferred way to go.

In Short

A Hang-On-Back filter is a perfectly viable and effective tool for many different kinds of aquariums. By shifting your mindset from the freshwater “all-in-one” approach to a more specialised support role, the Hang-On-Back filter becomes an excellent platform for water movement and targeted mechanical and chemical filtration.

While it can’t fully replace the functionality of a large sump, its simplicity, affordability, and ease of use make it an outstanding gateway into the captivating world of saltwater aquarium keeping, provided you commit to a rigorous maintenance schedule.

Convinced that a Hang-On-Back filter is the right choice for your simple reef? That’s great! Now that you know what to look for, our next guide will walk you through the exact steps to set it up, choose the best filter media, and maintain your hang on back filter for a thriving marine tank. That’s coming up very shortly. Thanks for reading and spending your time at Simple Reefs.