Why Internal Filters Don’t Belong in Marine Aquariums - And What Filters Would Be Better for Saltwater Conversions

Welcome to Simple Reefs. We’ve been talking a lot about converting freshwater aquariums into marine aquariums, as of late. It’s a topic I am quite passionate about and I’ll get into why in just a second. Today, we are looking at Why Internal Filters Don’t Belong in Marine Aquariums.

I started my saltwater journey with an old 260 litre freshwater tank. That’s all I had and, to be perfectly honest, people can make it seem like it is impossible to succeed with this kind of setup. Truth be told, there’s a ton of pretention and elitism in this hobby and a distinct “you have to follow these steps to have success” rigidity to people’s attitudes.

That’s completely untrue! Sure, you are going to have to make compromises when you convert a freshwater aquarium into a marine aquarium. But you can achieve success and have a beautiful reef. I did it myself! I had a tank full of fish, soft coral, LPS coral, and SPS coral. One of those places where I had to compromise was on filtration

Can I Use an Internal Filter for Saltwater

Let’s say you are making the leap from freshwater to marine by repurposing a freshwater aquarium just like I did. As you plan your conversion, you’re probably looking at your existing equipment, including that internal filter, and wondering:

“Can I just reuse this?”

It’s a natural thought. In fact, I did it myself with two large internal filters that my late father used in his tank. They were expensive and seemed to be able to shift a ton of water. Logically, it makes a lot of sense, right? If the filter kept your freshwater tank clean, why wouldn’t it work for saltwater?

The short answer is: while it’s tempting, internal filters are poorly suited to marine aquariums and will almost always lead to long-term problems, a lot of extra maintenance, and big declines in water quality. In the long run, they will end up costing you more money.

While internal filters do have certain applications in marine aquariums, which we will go into in a future article. They really aren’t going to be sufficient as your primary source of filtration. Let’s get into the reasons why that is.

But Why Can’t I Reuse My Internal Filter?

I don’t normally like to be so rigid when it comes to freshwater conversions. After all, a big part of my motivation for creating this website was to help people take their first steps into the wonderful world of marine fishkeeping while saving as much money as possible.

Let’s be honest, as well, what could save you more money than simply reusing the equipment you already have? With that being said, one rigid thought when it comes to marine aquariums that I do agree with is that internal filters don’t belong in marine aquariums.

There are far better options that are:

• Easier to maintain
• More effective
• Have more options for filtration media
• Are better suited to saltwater aquariums
• Less obstructive to your display

The truth of the matter is, internal filters aren’t capable of providing the levels of filtration required for marine fish tanks and reefs. Reefs require water that is extremely low in nutrients, namely nitrate and phosphate which come as a bi-product of the nitrogen cycle (NH₃ → NO₂⁻ → NO₃⁻​).

To attain these low nutrients, we need a filter that has enough space for plenty of filtration media and the ability to process a lot of water every hour. We need to be able to remove waste from the water column before it rots and we need to be doing it constantly and at a high rate.

Why Internal Filters Seem Like a Good Idea

Honestly, I get it! I have been there myself. Internal filters are a staple of beginner freshwater setups for good reason; they just work. They’re compact, inexpensive, and easy to install. Their main selling points are clear:

• Simplicity: Plug it in, and it works. No plumbing, drilling, or complicated setup. You don’t even need space behind your tank.
• Low Cost: Internal filters are among the cheapest filtration options on the market. Some are ridiculously affordable.
• All-in-One Design: You can have your pump, sponge, and media packed into a single box, saving tons of space and avoiding additional equipment.

For a freshwater setup, this is usually enough. The truth of the matter, however, is that your freshwater fish were far less demanding that the residents of a marine aquarium. Hell, unless you had some particularly delicate species of fish or certain plants, you probably never even gave much thought to the nutrients in your water.

We have to take the complete opposite approach with marine. As soon as we start including coral and invertebrates into our tanks, we have to become obsessed with nutrients, almost to an unhealthy level. That’s where internal filters will let you down over and over again.

The Saltwater Conundrum: How Internal Filters Become “Nitrate Factories”

The number one reason to avoid an internal filter in a marine tank is that it inevitably becomes what hobbyists call a “nitrate factory”. Trust me, you are going to hear this phrase an absolute ton when converting a freshwater aquarium into a marine aquarium. Almost to a frustrating degree. But what does it mean?

A “nitrate factory” is aquarium hobby slang for equipment that traps waste and turns it into nitrate instead of removing it. In an ideal setup, filtration media will trap the waste and we will remove it from the filter before it can rot. In a nitrate factory, that process doesn’t occur. The waste can’t be removed easily so it sits and rots.

Bits of leftover food and fish waste get caught somewhere in the equipment, where bacteria break them down through the nitrogen cycle. Nitrate is always the end result of this cycle, and in a saltwater tank it quickly builds up, fuelling algae growth and stressing corals. In other words, instead of helping clean your tank, a nitrate factory just traps it and makes the problem even worse.

Here’s why that is so likely to happen in an internal filter:

1. Waste Trapping: The sponge in your filter does a great job of collecting fish poop, uneaten food, and other organic debris.
2. Continuous Breakdown: That waste sits inside the filter, in oxygen-rich water, where beneficial bacteria thrive. They convert ammonia → nitrite → nitrate.
3. End Product Build-Up: Unlike freshwater fish, many marine species (especially corals and invertebrates) are highly sensitive to nitrates. While a bit of nitrate is tolerated, consistently high levels fuel nuisance algae blooms and stress livestock.

The result? A tank that looks fine for a few weeks, but quickly spirals into constant algae problems, frustrated coral growth, and extra maintenance. That’s without mentioning the trap of high cost, inefficient, proprietary filter media cartridge systems that you can easily find yourself in.

Internal Filter Nitrate Factory – Couldn’t I Just Clean The Filter?

Well, you could just clean the filter, change the media, and give everything a rinse. But you really want to be doing that a couple of times a week before the food and waste has a chance to be broken down and converted into nitrate.

With a hang-on-back filter, that’s a fairly simple task. Pop the lid off, pull out the dirty filter floss and replace it. Easy! It takes a couple of minutes. With an internal filter, that’s a much more drawn out and, frankly, frustrating task.

There’s no quick and easy way to clean an internal filter. You can’t just crack the filter open inside the tank because it will release the waste back into the water. You have to disconnect it, remove it, pull it apart, clean it, replace the media, then put it back in.

That’s a lot of hassle and a significant amount of disruption to your tank. I couldn’t imagine most hobbyists wanting to do this when there are far better options. On top of that, the longer we can go without putting our hands inside the tank water, the better.

This way, we are minimising the chance of upsetting our fish and avoiding the risk of introducing dangerous products that may be on our hands into the water. Everything in the marine fishkeeping hobby is about maintaining stability and this is one way to help with that.

Other Limitations of Internal Filters

Beyond the nitrate issue, there are other reasons why you should look for alternatives when it comes to filtration in a marine aquarium. Internal filters simply aren’t flexible enough for saltwater needs:

• Limited Media Options: Marine tanks often need chemical filtration (like carbon for clarity or GFO for phosphate control). Most internal filters have little or no space for this. You are sod out of luck if you want to include something like a Purigen pouch.
• Takes Up Tank Space: In a marine tank, every inch of display space matters, whether it is for more live rock, coral placement, or just swimming room. An internal filter hogs space and looks unsightly. It casts a shadow, as well, making a fair bit of your tank less than ideal for coral.
• Difficult Maintenance: As I mentioned above, to service an internal filter, you often have to put your hands in the tank, disturbing livestock and risking contamination. It’s a pain to actually perform this maintenance as many times as week as a marine aquarium would require, as well.

There’s another big one, as well, that I think a lot of people miss when talking about this subject. But what is that?

Flow Rate vs. Effective Filtration

Internal filters are, typically, less capable of filtering the water the amount of times, per hour, that your marine aquarium is going to require. I know what you might be thinking, and you are right. You can purchase internal filters that boast of being able to move large amounts of water per hour.

The truth is, this number can be extremely misleading. The issue isn’t just about the raw flow rate; it’s about the quality and capacity of the filtration that the water passes through. When it comes to marine, bigger is always better and that’s the problem.

In a marine aquarium, you need high water turnover (often 10-20 times the tank’s volume per hour) for two main reasons:

1. Filtration: Processing the water through biological and chemical media to remove waste.
2. Circulation: Creating strong internal currents for coral health, gas exchange, and keeping detritus suspended so that it can actually be filtered out.

An internal filter’s high flow rate is pushed through a relatively tiny sponge and a tiny, often optional, media chamber. This means the water has very little “contact time” with the filter media, resulting in inefficient filtration. But what does that mean?

What Is Contact Time and Why Is It So Important?

When we talk about contact time in filtration, we mean how long the water spends in contact with the filter media as it passes through. The longer the contact time, the more opportunity the media has to do its job; whether that’s trapping fine particles of uneaten food, allowing bacteria to process waste, or letting chemical media like carbon or phosphate removers absorb impurities.

I’m a big coffee fan so let’s think of it like making filter coffee. If water rushes through the grounds too quickly, you get weak, under-extracted coffee. But if it spends enough time soaking through, you get a full, rich brew. Or, for you tea drinkers, if you only leave the tea-bag in the water for ten seconds, there won’t be enough contact time with the water to actually infuse the tea.

💡 Pro Tip: The longer water stays in contact with your filter media, the more effective the filtration. Fast flow means less cleaning; steady flow gives bacteria and media the time they need to do their job.

The same principle applies to aquarium filters. A slow, steady flow through a well-packed media chamber is usually more effective than water rushing straight through. This is why filters with larger chambers, such as canister filters or Hang-on-Back units with media baskets, often perform better than internal filters.

They thrive at the things that internal filters struggle with. Effectively maximising both the volume of media and the contact time available for cleaning the water. In short, an internal filter doesn’t allow enough contact time with the filtration media for it to do its job. It moves the water around but doesn’t effectively clean it.

Best Filters For Saltwater Conversions and Sumpless Tanks

So, we’ve established that internal filters are a poor choice for a sumpless marine tank. The good news is, none of this means that you actually need a complicated sump to succeed in this hobby! Two excellent, affordable, and simple alternatives can carry you far beyond what an internal filter can manage.

When it comes to sumpless marine aquarium filtration, let’s talk about my personal favourite, first. As I mentioned earlier, I have had a ton of success with a hang-on-back filter but they are more suited for small to medium tanks.

1. Hang-on-Back (HOB) Filter

A favourite for marine hobbyists looking to convert a freshwater tank into a saltwater tank. A HOB filter hangs neatly on the back rim of the aquarium, keeping the unit out of sight but easily accessible for maintenance. You just need a few inches behind your tank to accommodate a hang-on-back filter.

Why Hang-On-Back filters work:

• Customisable Media Baskets: Pack them with your choice of filter floss, carbon, phosphate removers, and useful things like Purigen depending on your tank’s needs. It’s totally up to you what you use as filtration media.
• Easy Maintenance: Cleaning or swapping media is as simple as lifting the lid. No hands in the tank required, no clumsy disassembly. It’s just so easy.
• Extra Flow: Many hang-on-back filters return water to the tank through a waterfall like method known as a spillway. This breaks the water’s surface and helps oxygenate the water as well as providing additional flow for your coral and fish.
• Expandable: Many Hang-On-Back protein skimmers are designed to sit right next to your filter, making it easy to build a compact but powerful filtration system. I had a hang-on-back filter and a hang-on-back protein skimmer enabling me to keep delicate coral and maintain low nutrients.
• Versatile: Hang-on-back filters are not just versatile in the types of filtration media you can use in them. They can also be turned into refugiums for growing macro algae with just a few minor adjustments.

We’ve talked a lot about hang-on-back filters. If you are looking for an introduction and a handy beginners guide, simply check out this article right here.

2. Canister Filter

For larger tanks without a sump, a canister filter offers much needed additional power and flexibility. These external units sit in your cabinet and connect via hoses, leaving your display uncluttered. They also offer a hell of a lot of additional filtration capacity. Some are even modular and can be expanded with additional canisters containing UV filtration. I have seen people running thriving reefs in 180 gallon converted freshwater aquariums using canister filters.

Why Canister Filters work:

• Huge Media Capacity: Canisters can hold far more biological and chemical media than most other sumpless filters. You can even daisy chain some or buy additional canisters to increase filtration capacity. The possibilities are seriously impressive.
• Clutter-Free Display: Only a small intake and return pipe are visible inside the tank.
• Great for Larger Loads: Particularly suited to bigger tanks with heavy bioloads.
• Can Move a Lot of Water: Large canister filters are capable of moving a serious amount of water. They will generate decent flow and provide a lot of water turnover and increased contact time.

One important note: Canister filters can also become nitrate traps if neglected. In fact, they are pretty notorious for it and many marine hobbyists will scoff at the mere thought of using them. It can be avoided, though. Regular thorough cleaning (at least monthly) is essential to remove detritus and prevent nutrient build-up. Replace media often. We will be talking all about canister filters very soon.

Final Thoughts

Your old internal filter may have been perfect for a freshwater setup, but saltwater aquariums demand more… much more. Internal filters simply don’t provide the nutrient control, flexibility, contact time, or convenience needed for a healthy reef or fish-only marine tank.

By choosing a Hang-on-Back filter for small to medium tanks or a Canister filter for larger setups, you’ll give yourself a far better starting point. Add in live rock for natural biological filtration, and you’re setting the foundation for a stable, vibrant, and algae-free marine aquarium.

I want you to think of it this way: upgrading your filter now saves you months of frustration later. Not only that, but it is going to save you money in the long run, too. In the marine hobby, prevention is always easier than cure and it’s a hell of a lot cheaper, too. Trust me! Thanks for reading and spending your time at Simple Reefs.