Is Undergravel Filter Good for Planted Tank? – All You Need to Know

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There’s no doubt that a robust and effective filtration system can work wonders in an aquatic setup. The ideal filtration unit will benefit both fish and plants by improving water oxygenation, cleaning waste, food residues, and dead organic matter, and keeping the environment stable.

As you probably already know, several types of filtration systems are available, based on their role and construction. In this sense, we have canister filters, HOB systems, airlift filters, trickle filters, diatom filters, etc.

These products deliver different results based on your needs. Mechanical filters clean the water of larger floating particles and remove debris, bio-filters remove viruses, bacteria, and other pathogens, while chemical filters cleanse and dilute various toxins.

However, today we will discuss a more specialized type of filtration mechanism, and that’s the undergravel filter.

How does this work, and is it the best choice for your planted aquarium? Let’s have a look!

Are Undergravel Filters Good for Planted Fish Tanks?

I would say that the response is rather mixed. But it steers more towards no than yes. This doesn’t mean that undergravel filters are not good in general; it’s just that they’re not ideal for planted tanks.

This has to do with the filter’s very functioning mechanism. Undergravel filters consist of a flat plate that goes under the substrate, preferably covering the entire tank bed.

The substrate goes on top, which means that the filter sucks in water through the substrate. It filters it and then redirects it back into the tank via the output(s).

The main issue here is the suction aspect. Fish and plants may have difficulties in an environment where all the water is being constantly pulled through the substrate.

A conventional filtration system doesn’t have this problem because the intake is generally located in one area only.

With undergravel filters, the intake source covers the entire tank bed. We’ll discuss why this is a problem in planted tanks shortly.

Benefits of Undergravel Filter

As is normal with any piece of equipment, undergravel filters come with both benefits and downsides.

So, let’s check the benefits first:

– Provides Clearer Water

The filter sucks in smaller debris, food particles, and fish waste through the substrate, forcing them to stay there.

The suction power alone is sufficient to clear up the water and prevent the particles from floating. Regular filtration systems operate differently in this sense.

A canister filter sucks floating particles before they reach the substrate, but that’s about it. Once they get there, the filter can no longer get them unless the intake is close to the substrate.

And this isn’t exactly ideal due to the filter also sucking in large amounts of substrate in the process.

The undergravel filter doesn’t miss anything, as everything tends to come down. And the filtration system will trap all the particles in the substrate, keeping the water cleaner and healthier.

Naturally, this comes with the need for regular substrate vacuuming to prevent decaying organic matter inside the substrate, but we’ll get into that shortly.

– Large Surface Area

The core problem with most filtration systems is that they have a limited range. Most filters have improved efficiency in the nearby vicinity of their intake, but their effectiveness drops farther from it.

So, the filter won’t cleanse the tank uniformly, causing various dead zones to emerge in areas with insufficient water suction.

This is the main reason why many people use 2 filters on the opposite sides of the tank. Especially for larger aquariums that could benefit from the additional filtration power.

The undergravel filter removes the need for a secondary filtration unit. That’s because the piece covers the entire substrate, leaving no room for dead zones.

That being said, you may need 2 undergravel filters for larger tanks where one piece cannot cover the entire tank bed.

– Helps with Plant Growth

This may seem like a peculiar point, but it will make sense in a second, I promise. You typically have 2 types of plants, rooted and floating. The main difference between them, other than their overall behavior, is how they feed.

Rooted plants extract their nutrients from the substrate via their roots. Floating ones get their nutrients straight from the water column.

This means that the way you’re feeding your plants also differs. For rooted plants, you use root tabs which are fertilizing tablets that you will bury in the substrate.

These will eliminate nutrients gradually, providing your plants with steady nutrition over time. Floating plants require liquid fertilization.

Why do all these matter? Because the undergravel filter allows rooted plants to get their nutrients straight from the water column, the same as their floating counterparts.

They achieve this by sucking in the water through the substrate and bringing the nutrients to the plants’ roots.

So, yes, undergravel filters actually help plants grow faster and bigger. Naturally, there are some problems with how the undergravel filters operate in relation to rooted plants. We’ll discuss this one shortly.

– Good Water Oxygenation

Undergravel filters improve water oxygenation via the output system. This is a natural outcome of using any filtration system, so nothing new here. What is new is the filter’s effect on the beneficial bacteria colonizing the substrate.

These are aerobic organisms that require oxygen to perform their daily duties, such as converting ammonia into nitrites and then nitrates.

The undergravel filter sucks in oxygenated water through the substrate, providing the bacteria with all the nutrients and oxygen they need to thrive.

This is key to creating a stable and thriving aquatic setup, with a rich biofilm to support the system’s chemical balance.

– Cost-Effective Option

Undergravel filters are cheap, which is always a major plus. They’re also easy to install and maintain, although they’re not quite easy to clean. But we’ll get into that.

Downfalls of Undergravel Filter

Unfortunately, there are downsides to using an undergravel filter, causing many people to skip this one entirely. Here’s what I mean by that:

– Substrate Problems

There are several substrate problems worth mentioning here:

  • Can’t Use Sand – This should go without saying. Sand is too fine and will immediately get sucked into the filter, clogging the equipment on the spot. You can’t use soil either for the same reason or fine gravel, for that matter. So, you’re stuck with medium-large-sized gravel as your only option.
  • The Need for Intense Cleaning – You need to perform more substrate cleaning than normal. Many aquarists have reported excessive nitrate buildup when using an undergravel filter. This is due to the filter sucking all residues through the substrate. The larger ones won’t get inside the filter. Instead, they will remain buried in the substrate, decay, and produce higher amounts of ammonia and nitrates. So, you need to vacuum the substrate regularly to remove waste and prevent nitrate buildup, and algae spread.
  • Difficulties Cleaning the Filter – All filtration systems require cleaning and maintenance occasionally, which also stays true for undergravel filters. The problem is that you cannot access the filter without removing the entire substrate. This can be a serious pain in the case of planted and fish-filled aquariums. Cleaning the filter requires you to disturb the entire environment in the process.

As you can tell, these substrate-related problems are already enough to deter a lot of people from getting their own undergravel filter. Especially those who want to craft a more self-sustainable system with minimal maintenance work.

– Plant Roots Get Tangled in It

This is probably the most problematic trait of undergravel filters. You can have rooted plants and an undergravel filter in the same setup, but you need to be wary about the risks.

Rooted plants will anchor themselves quite deep within the substrate, which is why you need at least 2-3 inches of substrate, especially for larger plants.

This can cause the roots to get tangled in the filter, hurting the plants and clogging the filter in the process.

This causes many people to rely on floating plants instead. These are less prone to being affected by the filter’s activity.

– The Dangerous Buildup of Hydrogen Sulfide

Ah, the memorable odor of rotten eggs and foul cheese seeping from your water tank? Who doesn’t fear or loathe that?

Unfortunately, this problem is often related to the use of undergravel filters. The reason for that is the creation of dead zones inside the substrate, leading to matter decay.

The organic matter trapped in the substrate will decompose and eliminate hydrogen sulfide, which is toxic for fish.

Fortunately, the substance is neutralized when in contact with oxygen molecules in the water, but never entirely. The main issue here is that the problem will aggravate fast.

Soon, the hydrogen sulfide created in the tank will outweigh the system’s ability to defuse it. To fix this issue, you should clean the substrate regularly.

Substrate vacuuming, regular water changes, and gravel stirring are necessary to prevent the formation of noxious dead zones.

– Fish Can Get Trapped

This is another problem worth mentioning. Substrate diggers and substrate dwellers are at an increased risk of getting trapped in the substrate.

That’s pretty much a death sentence for fish since they will drown or starve in the process.

Naturally, the filter’s power makes all the difference here. So, you can have a fish-filled tank without experiencing this problem.

The issue is that, when it comes to larger tanks, you can’t really keep your filter’s power too low. Otherwise, the filter’s effectiveness will drop dramatically.

Smaller, bottom-dwelling fish are more prone to the risk of getting stuck under the substrate. So, avoid smaller loaches, blennies, gobies, and even plecos and other catfishes.

What Filter Media do Undergravel Filters Use?

Undergravel filters accomplish 2 important tasks: mechanical and biological filtration. So, these come with replaceable media cartridges, which become detrimental to your system over time.

Understanding how the system works will provide you with great insight into its mechanism.

The filter will:

  • Trap larger particles in the substrate – We’re talking about food residues, fish waste, organic plant matter, etc. Everything that the plants can use as food. These larger particles are too large for them to traverse the entire substrate’s depth. So, they will get trapped inside the gravel, decompose, and turn into plant nutrients.
  • Trap finer particles in the filter media – The finer particles traverse the entire substrate and get trapped in the filter’s media. These will feed the filter’s biofilm, allowing for improved biological filtration.

Regular cleaning is necessary to prevent the filter’s media from getting clogged, keep the substrate more well-aerated, and prevent algae and dead zones.

Do You Ever Clean and Undergravel Filter?

Yes, undergravel filters require cleaning to maintain their maximum performance.

A maintenance routine is necessary to preserve the system’s balance and keep the filtration system in good operating conditions.

Fortunately, the cleaning process can be quite straightforward, despite what you may hear on the internet.

Here are some steps to consider:

  • Vacuuming the substrate – Vacuuming the substrate will also clean the undergravel filter. That’s because the dirt will go through the gravel and get stuck on the filter’s bed. When you’re vacuuming the substrate, you’re dislodging those particles and muck that may accumulate on the filter, which are often responsible for excess hydrogen sulfide. Substrate vacuuming is generally sufficient to keep the filter in good operating conditions over time.
  • Cleaning under the filter – The problem with undergravel filters is that muck and dirt will also accumulate under them. This generally takes a long time, but it will happen. You can slow down the process by vacuuming the substrate regularly but, at one point, you will have to clean under the filter as well. You can do that once a year (generally speaking) by removing everything from the tank, plants, fish, decorations, substrate, etc. Then you remove the filter, clean the tank’s bed, and then move everything back in. It’s a cleaning operation of more massive proportions, but it’s necessary.
  • Using a canister filter – You can use a canister filter to clean the muck accumulating under your undergravel filter. The same process will unclog the filter in case there’s dirt stuck in the tubbing. The process is simple. You attach the canister filter’s intake to the undergravel filter’s output and turn the canister filter on. The suctioning power should be enough to unclog the filter and take in most of the muck accumulating under your undergravel system.

While these points may seem like a chore, they really aren’t. You only need to perform regular substrate vacuuming and water changes, and your system will remain stable and healthy. How often you should do that varies based on your system’s profile.

A variety of factors influence the frequency of the maintenance work, including:

  • The tank’s size and layout
  • How many plants and fish do you have
  • The type of fish you own, since some produce more waste than others
  • How much do you feed your fish because overfeeding creates more fish waste and food residues
  • The type of plants you have, given that larger, faster-growing plants produce more dead matter, etc.

Why are Undergravel Filters Obsolete?

I would say that most people avoid undergravel filters because of the difficulty of maintenance.

Undergravel filters are linked to the creation of dead hydrogen sulfide zones, excessive levels of nitrates in the water, and being almost incompatible with rooted plants and smaller fish species.

But I think that the filter’s main downfall is the difficulty of cleaning. You cannot perform an overall cleaning without removing everything from the tank, which can be a massive headache.

For this reason, those who insist on getting an undergravel filter usually do so for:

  • Hospital tanks – These don’t require any plants, because fish will only stay here temporarily. Hospital tanks are necessary when fish require to undergo a quarantine period, during which they receive medication and personalized treatment. The filtration system will require less cleaning because you don’t have as many fish or plants to create a lot of mess.
  • Breeding tanks – Undergravel-based breeding tanks are great for egg scatterers that don’t require any special type of substrate. Gravel or marble substrates are ideal for them, especially when paired with a slow-current undergravel filtration system.
  • Smaller tanks – We’re talking about 10-20 gallons at most. That’s because smaller tanks are easier to vacuum. The downside is that smaller tanks get dirtier faster than larger ones. So, you will probably need to perform regular cleaning more often. But, hey, you can’t have everything.

What is the Best Undergravel Filter for Planted Tank?

If you’ve decided that an undergravel filter is just what you need, consider this piece.

Lee’s 13210 Original Gravel Filter is resistant, effective, and affordable, making it great for low-tech tanks. It comes in 10-by-20 inches with 2 uplift tubes for improved personalization.

Conclusion

Undergravel filters aren’t exactly the most popular pieces in the world, but you can make them work. They are actually quite useful in low-tech systems with minimal amounts of plants and fish.

For fish tanks, avoid smaller species, especially bottom-dwellers, that could get trapped in the substrate due to the filter’s activity.

The situation is even more complicated with rooted plants, so I would recommend avoiding those altogether.

avatar I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.

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Questions and Answers

I enjoyed your article and learned a lot about my Under gravel filter which I’ve had in my 65 gal planted aquarium for almost 20 years now. I vacuum when I do a water change, keeping the gravel clean. I also have a canister and a box filter. I regulate with Co2 injection system.
I hear so many nays about using UGF so it was refreshing to hear some thing positive about them.
Thanks again
Bob

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