How to Set Up a Hang-On-Back Filter for a Thriving Marine Aquarium - An Easy Guide

Welcome back to Simple Reefs! If you are reading this, there is a good chance you’ve decided to use a Hang-On-Back filter for your marine aquarium. Congratulations, you’ve made an excellent choice for a simple, affordable setup. Today, we are going to be showing you How to Set Up a Hang-On-Back Filter for a Thriving Marine Aquarium.

You Can Have Marine Aquarium Success With a Hang-On-Back filter

As I have mentioned many times on this website. I actually started my marine aquarium journey with a freshwater aquarium that I converted into a marine aquarium. While doing this, I chose to avoid drilling the tank or risking leaks through a canister filter. Instead, I opted for a Hang-On-Back filter.

The aquarium was 260 litres and I purchased a Seachem Tidal 75, along with some other Hang-On-Back equipment like a protein skimmer. During my time running this setup, I had a ton of success. In fact, moving on to a tank with a sump, I sort of missed the simplicity of my Hang-On-Back filter. If I had success, you can have success too.

This guide will provide a step-by-step walkthrough of everything you need to do, from cleaning your filter to choosing the perfect media. (If you’re still not sure if a HOB is right for you, we recommend reading our Beginner’s Guide to HOB Filters first!).

Getting Started With Your New Hang-On-Back Filter

So we are assuming, here, that you have chosen your Hang-On-Back filter after reading our previous article (linked above) and are ready to get started. Don’t worry if you already own a Hang-On-Back filter or are repurposing a filter from your freshwater aquarium. This applies to you and there are some extra tips below for anyone coming from the freshwater hobby.

Before you do anything else, the first step with any new hang-on-back filter is a thorough rinse. There may be various oils or dirt on the filter, even when it is new, so we want to get that off. Unbox all the components and inspect them to ensure nothing is cracked or missing. You might be surprised how often parts can arrive damaged.

Take the main filter box, intake tubes, and any other plastic parts and rinse them well under lukewarm tap water to remove any manufacturing dust or residues. We want to make a decision here. If the filter came with any filtration media, do we want to use that or throw it away? If the media is in the form of cartridges that require specific replacements by the manufacturer itself, I would suggest not using them. Refer to the suggested filter media section below for more information.

If we are going to use the included media, pay special attention to it. If your filter came with a carbon pouch, it is essential to rinse this separately in cold water until the water runs completely clear. This prevents a fine, black dust from clouding your aquarium water when you first turn the filter on. Any sponges or biological media should also be given a quick rinse to remove dust. I like to do an extra rinse with RO/DI water just to be safe.

Once everything is clean, it’s time for assembly. Following the manufacturer’s instructions as every filter differs. Usually, setup will include connecting the intake tube assembly, ensuring the strainer is securely at the bottom. Attaching the motor to the filter body if it isn’t already, and placing the rinsed filter media into the media basket or chamber in the recommended order.

Packing The Filter and Priming

Typically, water flows through mechanical media (like a sponge or filter floss) first, followed by chemical media (carbon), and finally biological media though we recommend that you avoid that with marine aquariums. Again, check the suggested filter media section below for more information.

With the filter assembled, hang it securely on the back rim of your aquarium. Most models have a small levelling dial or foot at the base; adjust this so that the filter box sits straight and level, not tilted forward or backward. You will want to make sure it is nice and tight with no movement but don’t over-tighten it until the dial breaks.

The final and most critical step is priming the filter. Never plug in and run a Hang-On-Back filter while the motor is dry, as this can quickly burn it out. Some Hang-On-Back filters are self-priming and can pull water in immediately but that isn’t always the case. Don’t make any assumptions here. If it doesn’t advertise itself as being self-priming, assume it isn’t. Running a Hang-On-Back filter dry can result in severe motor damage and a voided warranty.

With the filter in place on the tank, take a cup and fill the filter box completely with water from your aquarium. Once it’s full, you can plug the filter in. The motor should engage, and you will see it start to pull water up the intake tube, creating a continuous siphon.

The water will then cascade from the outflow spout back into the tank. Check that the flow is strong and consistent and use the adjustable flow control, if your model has one, to set the desired rate. Your new hang-on-back filter is now installed and running.

How to Repurpose Your Hang-On-Back Filter for a Marine Aquarium

If you already own a Hang-On-Back filter, perhaps one repurposed from a freshwater system, you can still use it on your new marine setup. You just need to follow these steps before installing it.

The fundamental philosophy of filtration differs fairly significantly between freshwater and saltwater systems. In most freshwater setups, the filter is the primary hub for biological, mechanical, and chemical filtration. In a marine aquarium, especially a reef tank, the live rock and sand bed in the display tank serve as the primary biological engine.

Your Hang-On-Back filter, therefore, transitions from being the system’s core to a powerful, versatile support tool. Mechanical and chemical filtration become the primary concerns and that means changing things up just a little.

Here’s how to make the switch:

1. Thorough Cleaning: Before anything else, completely disassemble and meticulously clean your Hang-On-Back filter. Scrub the filter body, intake tubes, and impeller with a simple brush and hot water. Do not use soap or detergents. If you are finding that cleaning is tough, you can use a white vinegar or citric acid based solution but make sure it is pure white vinegar or citric acid with no perfumes or dyes. You must remove any residual medications, algae treatments, or organic build up that may have accumulated during the filter’s freshwater life, as these can be toxic to sensitive marine invertebrates.
2. Discard Old Media: Throw away all of the old filter media. The carbon cartridges, ceramic rings, foam, and bio-sponges from your freshwater setup are not suitable and can leach unwanted substances. You will start with a completely empty filter chamber.
3. Rethink the Filter’s Function: Your Hang-On-Back filter’s new primary roles in a marine tank will be:
   • Water Movement and Surface Agitation: Crucial for gas exchange, ensuring high oxygen levels and releasing carbon dioxide (CO2​).
   • Housing Chemical Media: Providing a convenient, high-flow area for media that polish the water, remove smells, and remove specific pollutants.
   • Light Mechanical Filtration: Removing large, free-floating debris before it settles and breaks down into unwanted elements like nitrates and phosphates.

Once you have completed these steps, you are ready to pack your Hang-On-Back filter with filter media but what type should you be looking at?

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Suggested Filter Media for a Marine Hang-On-Back Filter

Forget those lousy all in one cartridges from your freshwater days. We are breaking free of the restraints of corporate predatory design and entering the world of buying media separately. The best approach is to layer specialised media in media bags for easy removal and replacement. Let’s take a look:

1. Mechanical Filtration (First Stage – Water Enters Here):
   • Filter Floss: The best option for catching fine particles. It’s cheap and should be treated as disposable. Replace it every 2-4 days. You can buy this stuff in bulk on auction sites and Amazon.
   • Coarse Sponge: A reusable option, but it must be removed and rinsed thoroughly in discarded saltwater during a water change at least weekly. You want to make sure it runs clean after rinsing as trapped food and waste will breakdown into nitrates.
2. Chemical Filtration (Second Stage): This is where the HOB truly shines and offers some serious flexibility.
   • High-Quality Activated Carbon: Removes dissolved organic compounds, toxins, and odours, keeping the water crystal clear. It can also be used to remove medical treatments and chemicals that may have entered your tank through the air. Replace every 4 weeks.
   • Granular Ferric Oxide (GFO): Highly effective at removing phosphates (PO43−​), a primary driver of nuisance algae. Use as needed based on phosphate testing.
   • Other Phosphate Removing Media: You have a lot of options when it comes to phosphate removal and most can be used in a Hang-On-Back filter. Rowaphos is a good example.
   • Synthetic Adsorbents (e.g. Seachem Purigen): There are a bunch of different adsorbents designed for further polishing your water. One of my favourites is Seachem Purigen. An excellent all around choice for controlling ammonia, nitrites, and nitrates by removing nitrogenous organic waste. It can be “recharged” and reused using plain bleach.
3. Biological Filtration (Final Stage – Optional but with Caution):
   • Best Practice: Avoid it. It might be controversial but my advice is to not fill your Hang-On-Back filter with ceramic rings or bio-balls. Your live rock is a far superior biological filter that won’t trap detritus in the same way. These types of filtration media seem to get clogged up very easy, especially when packed at the bottom of media baskets.
   • If You Must: If you are desperate for the extra biological filtration real estate, I would suggest a porous block like MarinePure. They tend to hold less debris and detritus but it still requires regular cleaning around it to prevent build up. A good rinse in used tank water should help, as well. For most HOB applications, it’s better to leave this space empty to improve flow and circulation to the other media.

I have used a number of different Hang-On-Back filters and absolutely loved them. I found them versatile, easy to clean, and very effective. I’ll give you an example of how I used to pack my Seachem Tidal 75 Hang-On-Back Filter.

My Hang-On-Back Filter Media Setup:

This was how I used to setup my hang on back filter on my 260 litre freshwater conversion. I found this to be very effective and it resulted in clean water with low nutrients:

1. Water flows into the HOB.
2. It first passes through a decent amount of filter floss. Cheap, effective, and easy to replace.
3. Next, it flows through a media bag containing activated carbon. I preferred Fluval’s carbon as it comes in a handy tub, is very affordable, and includes a media bag.
4. Next it passes through a Purigen pouch. Purigen can be purchased already packed in a media bag that is tightly woven enough to not leak.
5. Finally, it passes through another media bag containing ROWAphos to help remove the excess phosphate from the water.

I would replace the filter floss every few days and give the filter a thorough clean every time I did a water change. This would involve scrubbing the inside to get rid of detritus and rinsing the various bags of media in tank water. It was incredibly simple to maintain and my water was impressively clean.

Things to Consider When Using a HOB Filter

As you can probably see, there are loads of benefits to Hang-On-Back filters and they might just be the ideal solution if you are looking to convert a freshwater aquarium into a marine. To successfully use a Hang-On-Back filter in a marine environment, keep these points in mind:

• Go Big: Always oversize your Hang-On-Back filter. A filter rated for a 50-gallon tank is a good choice for a 20 or 30-gallon marine setup. The extra flow and media capacity are invaluable and you will be thanking your foresight when you can double up on filter media.
• Prioritise Flow: Your total tank turnover should be at least 5-10 times the aquarium’s volume per hour. While the Hang-On-Back filter contributes, you will almost certainly need one or more powerheads (wavemakers) inside the tank to provide the broad, varied flow that corals and fish require. That’s no different from having a sump, though.
• Maintenance is Non-Negotiable: Diligent maintenance is the paramount key to success. You must commit to cleaning the mechanical filtration media at least once or twice a week to prevent it from becoming a source of nitrates. This is why filter floss is so good. Pull it out, throw it away, and slap some new floss in.
• Modify for a Purpose: Don’t just use the default cartridges, they are a con designed to get you to spend more money. Think of the Hang-On-Back filter as a customisable media reactor. Use media bags to contain your chosen chemical filtration media and get smart with your setup.

In Short

And there you have it! Your Hang-On-Back filter is now a powerful, customised filtration engine for your marine aquarium. Remember, the key to success is diligent maintenance. By keeping your mechanical media clean and choosing your chemical media wisely, you can achieve crystal clear water and a healthy environment for your fish and corals. We will be talking all about maintaining your hang-on-back filter in an upcoming article. For now, welcome to the wonderful world of simple reefing!