How to Clean a Hang-On-Back Filter for a Pristine Marine Aquarium - An Easy To Follow Guide

Welcome to Simple Reefs, where we’re all about keeping reefing simple and affordable. We often talk about converting freshwater tanks for marine use, which brings us to a key piece of equipment: the filter. For a sumpless tank, one of our favourite solutions is the humble hang-on-back (HOB) filter.

Today, we are going to be talking all about How to Clean a Hang-On-Back Filter for a Pristine Marine Aquarium. If you are looking for more information on hang-on-back filters, why not check out our beginners guide to get yourself all caught up?

The Benefits of a Hang-On-Back Filter

When you convert a freshwater aquarium into a marine aquarium, you need effective filtration. In a typical reef setup, a sump handles mechanical filtration with filter socks or floss. Without a sump, a Hang-on-back filter is arguably the best tool for the job.

It provides essential mechanical and chemical filtration in a compact, simple, and easy-to-access package, preventing food and waste from decaying in your tank.

Unlike canister filters, which can be difficult to clean and become “nitrate factories,” a HOB filter is easily maintained. Media baskets are quick to remove and the filter is typically in easy reach with no clumsy pipes that need disconnecting and reconnecting. That brings us nicely onto the top of today’s article.

How to Clean a Hang-On-Back Filter

While cleaning a hang-on-back filter is important, improper cleaning can do more harm than good. Potentially introducing unwanted cleaning products or disrupting the delicate nitrogen cycle that is the lifeblood of any saltwater ecosystem. This is particularly true if you are using biological media in your filter.

The goal of hang-on-back filter maintenance is not to make it sterile and completely spotless, but to remove restrictive gunk and replenish exhausted media. Ultimately, our goal is to keep the filter functioning perfectly while preserving the vast majority of beneficial bacteria.

We are first going to look at simple hang-on-back filter maintenance before looking at how, and when, to clean your hang-on-back filter. Let’s kick off with a quick look at how often we should be cleaning and maintaining our filter.

How Often Should You Clean Your HOB Filter?

This question has two answers. One focuses more on the frequent light touch maintenance tasks and the other focuses on the less frequent full clean.

• Quick Maintenance: Simple tasks like changing filter floss can be done every 2-7 days, depending on your tank’s needs and the amount you feed.
• Full Cleaning: A complete disassembly and deep clean should be performed every 2 to 4 weeks, or whenever you notice a significant decrease in water flow.

Quick & Frequent Maintenance (The 5-Minute Tune-Up)

For aquarists aiming for pristine water clarity and low nutrient levels (nitrates and phosphates) which, let’s be honest, is almost all of us. Performing quick maintenance a few times a week can make a huge difference. These tasks focus on removing physical waste before it has a chance to break down and pollute the water.

As I have said many times on this website, I started my marine aquarium journey with a freshwater aquarium conversion. I opted to go with a hang-on-back filter (Seachem Tidal 75) and this was, basically, my routine every few days. It was easy to do, only took a minute, and resulted in water that had incredibly low nutrients. What more could you want?

• Changing Filter Floss: I tried a lot of different types of filter material before settling on filter floss. It’s just so damn cheap, comes in big rolls, is easy to dispose of, and traps masses of waste. Its primary job is to trap fine particles and essentially “polish” the water. Because it clogs quickly due to being so good at trapping waste, it’s highly effective when changed frequently.
  • How To: Simply open your filter, pull out the dirty wad of floss, and replace it with a fresh piece. It takes less than a minute.
  • Frequency: In a heavily stocked tank or a sensitive reef, changing it every 2-3 days is ideal. For most other tanks, once a week is sufficient. I had a fair few fish so I did it multiple times a week.
  • Benefit: This is one of the easiest ways to export solid waste and nutrients from your aquarium. Bin them before they rot.
• Rinsing Mechanical Sponges: Let’s be honest, there is an ethical concern to using filter floss. It is mostly synthetic and that means it won’t biodegrade. With this in mind, you may prefer to opt for a mechanical sponge that is reusable and should last a long time. If your first stage of filtration is a reusable sponge instead of disposable floss, giving it a quick rinse is very beneficial.
  • How To: During a water change, or by scooping a cup of tank water into a small container, take out the primary mechanical sponge. Give it one or two quick, firm squeezes in the tank water and a bit of a shake to release the loosest, heaviest debris. Then, place it right back in the filter.
  • Frequency: Once or twice a week.
  • Important: This is not a deep clean and it isn’t as effective as replacing filter floss. The goal is just to dislodge the gunk that is most likely to restrict flow, without disturbing the bacteria within the sponge’s deeper pores. Always use tank water for this rinse.
• Rinsing Carbon or Chemical Filtration: The principal here is exactly the same as with the sponge. You are just removing any bits of waste that have attached themselves to the media bag or have gotten trapped. This probably won’t be necessary as you will be changing the media out before it gets too dirty but I have found myself needing to do this before after heavy feeding.
  • How To: During a water change, or by using RO/DI, take out the chemical media bags and give them a shake in the water and then a quick rinse to release the loosest, heaviest debris. Then, place them right back in the filter.
  • Frequency: Whenever you notice that you have waste accumulated on your chemical media bags.
  • Important: You don’t want to introduce chemicals into your tank. Always use Tank or RO/DI water for this rinse.

That’s the most basic way to keep your hang-on-back filter removing as much waste and food from the water as possible. Every few weeks, however, we are going to want to go a little deeper than that.

The Full Cleaning Process (Every 2-4 Weeks)

While frequent tune-ups are great, a full deep clean is still necessary to keep the filter motor and other components running efficiently. It used to be extremely surprising how much waste could accumulate under the filter media and into the motor of my hang-on-back filter. Even with it having a self cleaning impellor.

These steps are going to keep your filter running perfectly and may even be essential for warranty claims should the worst happen. Always time up cleaning your hang-on-back filter with performing a partial water change. You are going to need some of the tank water to aid in the cleaning process.

What You’ll Need

Before you start, gather your supplies to make the process much smoother and quicker. I tend to keep all of my cleaning supplies in a bucket ready to use.

• A clean bucket, used only for aquarium maintenance.
• Some of your tank water.
• Old towels to place under the filter to catch drips. I also like puppy pads for this task.
• A filter brush or an old, clean toothbrush. Tesco sells them for less than 60p a pair.
• Replacement chemical media (e.g., activated carbon, GFO, ROWAphos).
• Replacement mechanical media (e.g. filter floss), if you use disposable pads.
• A small container to hold your biological media if you have it.
• Dog Waste bags (totally optional).

Step 1: Preparation is Key

Turn off and unplug the filter. You never want to run a filter motor dry as this can cause it to burn out. Trust me, that sound is not something you want to hear and it can quickly invalidate a warranty. Place your towels on the floor or cabinet below the filter to protect against saltwater spills.

I really like to include a puppy pad when I do this type of job as I rent and really don’t want a soaking wet floor. They are designed to absorb dog pee while toilet training so can hold a lot of fluid. I reuse them many times before replacing, as well.

I should probably point out, as well. If you think this process may take awhile, you might want to point a wavemaker at the surface of the water to agitate it a little. This will help inject additional oxygen into the water for your fish. Remember, your filter won’t be there to do this task while you are cleaning it.

Step 2: Remove and Disassemble

Carefully lift the hang-on-back filter off the back of the aquarium and place it on your towel-covered work surface or carry it to a sink. Be very careful not to give the plug a dipping in the tank water or sink. If this happens, I highly suggest cleaning with contact cleaner and grabbing a hair drier to help dry it out thoroughly.

Disassemble the filter, laying out each component: the intake tube, the media basket(s), the impeller housing, and the main filter box. Your hang-on-back filter should come with instructions on how to do this as it is a necessary part of maintenance. Follow those instructions for this step.

Step 3: Cleaning Your Filter Media

Your filter media is home to some of the beneficial bacteria that convert toxic ammonia and nitrite into less harmful nitrate (NH3​→NO2−​→NO3−​). Never use untreated tap water to clean your biological or mechanical media. The chlorine and chloramines in tap water will kill this bacteria and may leave your tank missing some of its extra biological filtration capacity.

1. Siphon Some Tank Water: This is why we say to clean your filter during a routine water change. Siphon a gallon or two of the old tank water into your clean bucket. This is the water you will use for cleaning.
2. For People With Mechanical Media (Sponges): Place your sponges in the bucket of tank water. Firmly squeeze and swish them around until the trapped debris is released. The water will turn brown but don’t worry, this is normal! The goal is to remove the physical waste, not to scrub the sponge back to its original colour.
3. For People With Filter Floss: Simply throw out the old filter floss and replace it with fresh. This is why I like to include dog waste bags. You can place the old floss in these scented bags, tie it up, and throw it in the bin to avoid foul smells. This is especially useful if you have infrequent bin collection like we do. Trust me, your partner will thank you.
4. Biological Media (Ceramic Rings, Bio-Balls): We don’t always need to include biological media in a hang-on-back filter that is used for marine aquariums. The sand and live rock should provide plenty of real estate for biological filtration. If you do have biological media, however, this media should be touched as little as possible. Place it in a small container with some tank water. Gently swish it around to dislodge any loose debris and then set it aside. Never scrub or aggressively rinse your bio-media, keep it well away from tap water.
5. Chemical Media (Activated Carbon, GFO, etc.): Chemical media becomes exhausted over time and cannot be “cleaned.” It must be replaced. A common schedule is to replace carbon every 4 weeks. Other chemical media may have different specified lifespans so check the instructions. Discard the old media pouch and replace it with a new, pre-rinsed one. Rinsing is important to remove dust but remember to do your final rinse with RO/DI water. For other chemical media, follow the manufacturer’s instructions on whether or not to rinse.

Step 4: Cleaning the Hardware

With all of your media taken care of, it’s time to clean the actual filter itself. This should be a fairly simple process. We don’t need to scrub the hell out of it, we just need to remove the more obvious waste stuck to it.

• Intake Tube & Filter Box: Use a filter brush or toothbrush to scrub away any algae or detritus from the inside of the intake tube, the media baskets, and the main filter box. It’s generally safe to rinse these hardware components with tap water but I always like to give them a rinse with RO/DI to finish up. You may want to remove Coralline accumulation as it can eventually clump up and block intake tubes.
• The Impeller Assembly: This is a vital and often overlooked step. A dirty impeller reduces flow and can cause horribly annoying rattling noises. Carefully remove the impeller cover. Gently pull out the impeller magnet. Use your brush to clean the impeller blades and the inside of the impeller well. Be sure not to drop anything down the sink. I have actually done this before with an impeller shaft and taking off the waste pipe on my sink was not something I was planning to do that day.

Step 5: Reassembly and Restart

Now that everything is clean, it’s time to get it all reassembled and back in the tank.

1. Put all the cleaned components back together.
2. Place your refreshed media back into the media baskets. Mechanical filtration media like floss usually goes nearest the intake so that the water hits it first, biological media goes in the middle, and chemical nearest to the where the water returns to the tank to polish the water before it leaves the filter but it may vary depending on your specific filter. Some spillway filters require a slightly different alignment so follow the manufacturer’s instructions.
3. Place the filter back onto the aquarium.
4. Prime the Filter: Before plugging it in, use a cup to fill the filter box with water from your aquarium. This “primes” the motor. Not all filters require this step but it doesn’t hurt to be safe rather than sorry.
5. Plug the filter back in. It should start quietly and produce a strong, steady flow of water. If you have a large filter, make sure none of your media bags or floss escape and end up in the water. This has happened to me in the past.

In Short

By combining frequent, light maintenance with periodic deep cleans, your hang-on-back filter will remain an effective and reliable component of your marine aquarium’s life support system. Decent filtration is one of the key ingredients in a successful reef. Just because you don’t have a sump doesn’t mean you can’t have a thriving saltwater tank. Thanks for reading and spending your time at Simple Reefs.