First Tank Guide Tank Logo
First Tank Guide Home

The Under Gravel Filter Controversy

Truth About the Reliability and Safety of Under Gravel Filters

(The First Tank Guide)

Some people and businesses on-line have been raising a big fuss about the 'undergravel [sic] filter controversy.' What is the under gravel filter controversy? The controversy is about whether or not under gravel filtration is a good method for filtering your aquarium water - and, in fact, if the under gravel filter is dangerous to your fish and the other aquatic life in your tank. Even some local pet shops will incorrectly tell people that under gravel filters are unsafe or are outdated technology.

So, What's the Problem with Under Gravel Filters?

The argument against under gravel filters is that over time, the gravel gets clogged with debris and flow rates decrease, thereby decreasing filtration and allowing a buildup of waste in the water, reducing the water quality. Furthermore the restriction in water flow through the gravel will allow anaerobic pockets to form in the gravel, which can promote the colonization of harmful bacteria which can potentially poison an entire tank. Additionally, an under gravel filter provides only biological filtration and poor mechanical filtration, and can only provide chemical filtration for short periods, and at the cost of reduced efficiency of both the biological filtration and mechanical filtration provided. In some of the extreme cases reported, loss of power for a few hours has resulted in entire tanks crashing, killing all the occupants of the tank!

Are These Problems Unique to Under Gravel Filters?

As you might suspect, these anaerobic pockets can form in the gravel of tanks with other types of filtration, and, in fact, they are more likely to because the filtration system is not pulling oxygenated water through the gravel. In any tank, regardless of the type of filtration used, cleaning the gravel (if any gravel is used in the tank) and providing regular, frequent, small water changes is very important to maintaining a healthy tank. As cleaning the gravel in a tank with or without an under gravel filter is necessary to maintain a healthy tank, and this is the only maintenance (besides cleaning the power heads or replacing the air stones as appropriate) that an under gravel filter needs, while, in addition to this maintenance, any other filter will have cartridges to replace, and often tubing that will need to be cleaned, you can see that the maintenance of an aquarium with an under gravel filter will be consistently lower than the maintenance required of a tank with most other types of filters.

What Are the Real Drawbacks of Under Gravel Fitlers?

Now, it is true that under gravel filters do not provide high-quality mechanical filtration, and that the use of the carbon or resin inserts for the lift tubes will only provide short-term chemical filtration and will greatly reduce the flow through the system, thereby reducing the effectiveness of the filter overall. Furthermore, if your under gravel filter is run with power heads mounted at the top of the lift tube, rather than with air stones and an air pump, you do not even have the option of inserting these carbon or resin cartridges, thereby eliminating the option of even partial or temporary chemical filtration through the under gravel filter.

But Under Gravel Fitlers Are Safe

When proper maintenance of the under gravel filter is provided, the under gravel filter is a proven, long lasting, inexpensive, safe, reliable, and low maintenance filter. The maintenance that an under gravel filter requires is regular cleaning of the gravel. I recommend that between 1/4 and 1/3 of the gravel be cleaned with a gravel vacuum with each weekly 10-15% water change. This will remove the debris from the gravel and prevent the gravel from getting blocked up, allowing free flow of water through the gravel and preventing the development of anaerobic pockets that can harbor harmful bacteria.

So, How Do People Support Their Complaints About Under gravel Filters?

Though I do not doubt the claims that these individuals have experienced losses while using under gravel filters, the nature of the losses indicate that the tank was poorly maintained and probably overpopulated to begin with. That the tanks were overpopulated is undoubtable as well as unsurprising, as these reports are about tanks in a pet shop, where the normal tank population is typically 5 to 10 times (or more) a safe load for the tank. The opinion that these tanks were poorly maintained is supported by the further evidence provided by the individuals making the claims that an under gravel filter is unsafe and unmaintainable. After switching from under gravel filters to other types of filtration, they also switched to providing more regular water changes and otherwise better tank maintenance. I would take the switch to more reliable and more frequent water changes and to providing the maintenance needed for the tank and equipment as a more legitimate and more reproducible solution to the problems that these individuals experienced. I have also experienced and heard about these types of catastrophic failures on tanks, but I have never had a failure like this on a tank with an under gravel filter, though I have seen failures like this with other filter systems. In every case, the failure could be tracked down to poor maintenance and/or overpopulation.

In order for the claims of these individuals to be reasonable and sustainable, first, the results would have to be reliably and consistently reproduceable. As I know of many individuals that use under gravel filters with great success and with few or no problems, and compounding this with the fact that under gravel filters still get great reviews and high marks on the necessities for fishkeeping from many experts in many different areas, we can be assures that these results are not reproduceable reliably if the filters are maintained. Furthermore, the claims that these massive problems are caused by debris blocking the gravel in tanks with under gravel filters, in order to prove that it was the under gravel filter causing the problem two things would have to be true. First, when using filters other than under gravel filters, there would have to be nothing to get plugged up, but in fact, there are filter cartridges or other filter media that gets plugged up. The difference is that in most cases, instead of cleaning the media when it gets plugged, the dirty media is just thrown away and replaced (this, of course, can cause its own problems...). Secondly, in order for this to be true, the gravel would have to stay clean and free of debris, and thoroughly oxygenated in tanks without under gravel filters. Though I cannot speak to the point of oxygenation, as I have never tested, nor do I have the means to test the water in the gravel of a tank for dissolved oxygen content without disturbing the gravel, it would seem logical that if oxygenated water is being pulled through the gravel by an under gravel filter, that the gravel is likely to have a higher dissolved oxygen content than if the water is not being circulated through the gravel. However, as far as the gravel staying clean and free of debris, I have yet to encounter a tank that did not have debris caught in the gravel one week after the gravel was cleaned - regardless of the type of filtration provided.

But Under Gravel Fitlers Are Safe

After a logical and rational assessment of the claims made by those who claim that under gravel filters are unsafe or useless, it is clear that the problems with that have been seen with these filters in some isolated situations were purely due to poor maintenance of the filters or an absolute refusal to provide the maintenance that is necessary for the tank to remain healthy.

There is no doubt that under gravel filters are not perfect. They do not provide chemical filtration, and only provide limited mechanical filtration. Under gravel filters are not the only method of filtration, and they are not appropriate for every type of tank or for all fish - but these statements can be made for any filtration system. However, when properly maintained, under gravel filters are reliable, long-lasting, inexpensive, low maintenance, and safe, and for these reasons they make a great filtration system for most beginner tanks.

"Thank you so much for the great advice!!!!!"
January 18, 2003
More Comments
"Thank you for taking the time to build this site. It is very informative and useful."
March 27, 2015
More Comments