What is an Aquarium Sump Filter?

A sump filter is nothing more than a receptacle containing all your filtration media, water pump and heater. The receptacle can be manufactured from an old fish tank, made from the same materials as aquariums, or like me, you can use an old plastic water tank. A sump filter can be extremely simple in design, or you can design an elaborate filter that utilises different types of filter media that all perform their own specific job.

Traditionally, sump filters were used on saltwater tanks where good water quality is essential. Nowadays as tanks get bigger and people start keeping much larger fish, sumps are being used more and more on freshwater aquariums because of the superb efficient filtration they offer.

There are a few different designs of the sump that vary in the way they are installed on the aquarium. Some of the designs are more complicated than others and will also involve drilling holes in the tank. The design I prefer requires you to purchase a tank that is already designed to have a sump filter installed on the aquarium. To be absolutely honest with you, unless you are fairly experienced and know your way around an aquarium, you may struggle to fit a sump filter by yourself. Worry not, if you really want one then there are plenty of people who will be able to do it for you.

Do you need a sump filter

There is nothing written in stone that states what the minimum size aquarium a sump filter should be installed on. However, you should ask yourself whether it's worth all the hassle installing a sump filter on an aquarium that can only hold a few fish. One of the main reasons we use sump filters is because they are able to accommodate a lot of media. They also enable you to add some extra water to the system. But remember that the fish will be living in the main tank and this will obviously restrict you to how many fish you can have. So there's little point in packing in loads of filtration if you can only house a small number of fish.

I've had three sizeable aquariums in the last 15 years. The first was 75 gallons, then I got myself a 125 gallon, then in 2007, I had a 300-gallon custom made aquarium made for me. I wouldn't have even contemplated installing a sump on the 75 or 125 gallons, canister filters provided more than enough filtration on these two aquariums. However, the 300 gallon was a different ball game altogether. In order for me to have filtered this properly with canister filters, I would have needed at least two very large canister filters. I decided that a much cheaper option would be to have the tank designed to take a sump filter. If I remember rightly, the sump actually costs £40 to manufacture. Obviously, you can add a bit more money on for the pumps and media, but it would have still been a lot cheaper than paying £350 for each canister filter.

Fitting a sump to your aquarium

Fitting a sump to an existing aquarium is probably the most complicated part of installing it. In order to fit all the pipework that allows the water to flow between the aquarium and sump, you may have to drill some holes in the main aquarium. Obviously, you don't want to start drilling holes in an aquarium that already has all the fish in it. Therefore the tank really should be prepared for a sump before any fish are added.

Unlike traditional filters like canisters that use a pump to remove water from the aquarium, a sump filter relies purely on gravity. In order for the water to drain from the main tank, you use a device called an overflow box. The overflow box is connected just inside the aquarium and acts very much the same as a weir. When the water level reaches the lip of the overflow box it will start flowing into this isolated box. Located inside the overflow box will be an outlet in which the water will flow through down to the sump. There are various ways to fit the overflow box to an aquarium. You can get overflow boxes which straddle the tank. The advantage with these type is there is no need to drill any holes in your aquarium. Some people prefer to drill a hole in the tank and attach the pipework in which the water will flow out of. My preferred method is to have the overflow box built into the aquarium. My 300-gallon aquarium has got two overflow boxes located at each end of the aquarium. The diagram below shows you the layout of how my overflow boxes are built into my aquarium.

What goes in the sump

The beauty of using a sump is you can make it self-contained. Basically, not only can it hold the filtration media, but it can also house your heater and pump as well. This is particularly advantageous because Oscar fish are notorious for breaking heaters.

How a sump filter works

A sump filter is normally divided up into sections that we call chambers. The chambers are created by using dividers that perform a very important task and therefore must be installed properly in order for the water to flow smoothly through the filter.

I have made a little diagram showing the basics of how a sump filter works. Apologies for my rather amateur drawing, I'm not an artist so this is just a basic diagram of which I hope you will be able to follow. As you can see water enters chamber number one. In this example the first chamber contains a mixture of biological and mechanical filtration, several hundred bio balls have been put in the chamber first, these are then topped off with several layers of mechanical sponge filtration. The water then flows down through the mechanical and biological filtration and enters the second chamber via a gap under the divider. Finally, the water flows up through biological bio balls and over the second divider into the third chamber which contains a pump and heater in our example. It's important to understand that if the second divider is not lower than the first divider then the water will not flow over into the third chamber, it will simply flood the whole filter. The final chamber that contains your pump must be designed to hold plenty of water, the bigger the tank, the more water it needs to hold.

Let's explain a little more in detail. The first two chambers are governed by how much water is in each, in other words, if you drain the first chamber, the second chamber will also empty because they are essentially connected by that small gap under the first divider. However, the third chamber that contains your pump is separate from the first two chambers so you could completely empty the first two chambers of water and the third chamber would not be affected at all. When you use a sump filter it's very important that you keep a very close eye on the water contained in the last chamber. There are two extremely important reasons why a close check needs to be kept on how much water is in the last chamber. When you switch your pump off water will continue to drain from your aquarium into the sump until the level drops just below the outlet pipe, once this happens the water will stop entering the sump from your aquarium. When the pump is switched back on again it will start taking water from the sump, the water will continue to drop in the last chamber until the water level reaches the outlet pipe and starts to drain out back to the sump. Once this happens the water level will stop dropping in the sump filter because the water will now be circulating through the whole system. Therefore you must always make sure you have got enough water contained in the last chamber, if you don't have enough water then you will just get to the point where the water level will drop below your pump and everything will just come to a halt. On the other hand, if you have too much water in your sump filter when you switch the pump off, you may well flood your room because there won't be enough space to accommodate the extra water that continues to flow from the tank when the pumps are switched off. It is common practice to place a mark on the filter which indicates the lowest point at which you want your water to lay at in the sump filter. You will also want a mark that indicates the maximum amount of water you will allow to enter that last chamber. The easiest way to fill the last chamber up to a level that is safe is to do it via the main aquarium. So instead of putting water into the sump itself, let water drain into the sump from the aquarium, when you get to a level you are happy, you will know that it won't get any higher because when you switch the pump on, the water will drop.

One very important thing I must mention is that you mustn't submerge the very end of your outlet pipe, the part where the water comes out of and enters your tank, if you submerge it completely then you run the risk of starting a siphon when you switch the pumps off. If this happens water will continue to syphon until the level of the water drops below these pipes, by this time you will have probably flooded your room.

One of the problems you get when having your outlets positioned above the water is they can sometimes create quite a lot of noise, especially if you are using powerful pumps. One way of reducing the noise dramatically is to submerge your outlets. So if you would prefer to position the outlet so it is completely submerged then to avoid creating a syphon once the pump is switched off drill a hole approximately 5 or 6 mm in the outlet pipe close to the surface. As soon as the water level reaches the hole, air will be drawn them and it will break the syphon. Another method which is commonly used with canister filters is to use a spray bar. Because spray bars are normally positioned just above the surface of the water, you will never experience a problem with creating a syphon when you turn your pumps off. Spray bars are particularly effective as they can create a very good surface agitation. You can also use spray bars to simulate rainfall which is a method that can trigger fish into spawning. If you do submerge your outlets completely then keep a very close eye on your fishes behaviour, if you see any signs of oxygen depletion then you may have to position your outlets so you are creating surface agitation. If there is quite a lot of movement on your system then hopefully plenty of oxygen is should be created, but it is something you will need to be aware of.

Using Standpipes to Reduce Noise

standpipe in overflow box on sump filterIn order for water to flow to your sump filter you've got to have some way for it to exit your aquarium. Canister filters use pumps to suck the water from the aquarium and then return it back again. Although sump filters utilise a pump in the system, this is only used to return the water back to the aquarium, water exits the aquarium using gravity. There are various different ways that you can set up an aquarium so it can house a sump filter, however, one of the more common ways is to use overflow boxes. Overflow boxes are normally installed inside the aquarium. Depending on the size of your tank, you can either have one overflow box installed or two. If you look at the simple diagram above you can see that this illustration shows two overflow boxes located at the rear of the aquarium. The white circles depict where the inlet and outlet pipes are located. The overflow box is designed so it's not quite the height of the aquarium glass, it needs to be like this so that the water flows freely into the overflow box like a weir. The water then flows through the inlet pipe and down into the sump filter, it is then returned through the outlet pipe. When you switch your pump off, the water will continue to flow into the outlet box until it drops below the lip of the weir, at this stage water will stop flowing out of the aquarium. There really isn't any more to it, this is how water exits your tank when using an overflow chamber or box. However, one of the drawbacks with using overflow boxes is they can be very noisy. If your aquarium is located somewhere where you spend lots of time then the sound of flowing water could well get rather annoying after a while. There is a simple solution to this problem and it can be resolved by using standpipes. You can either make them yourself, or you can buy commercially available pipes. When you locate standpipes in your overflow boxes, the water then backs up until it reaches the point where it will start flowing through the standpipes. Because the inlets on the standpipes are located near the top of the overflow box, the water doesn't fall so far, in fact you can set them up so the water only needs to for a couple of inches, this will dramatically reduce the noise the water makes when dropping, in fact it can cut the noise out altogether.


This diagram illustrates how you can include a trickle filter into your sump filter. This example still exhibits the characteristics of a standard sump filter. The first two chambers to the right contain both biological and mechanical filtration. However you will notice the third chamber is only partially filled with water, this is the basis of a trickle filter system. Most filters are full of water when in operation, therefore the biological media is always submerged in water, it gets its oxygen from the water flowing through the media. A trickle filter system operates in the opposite way, rather than being submerged underwater, the biological media is actually open to the air, it Is kept wet by water trickling down through the media, some people call them wet dry filters, however, these aren't strictly wet dry, wet dry filters act in a way that the media is submerged underwater, and then the water empties and exposes the media to air. So because the media is basically open to the air, it is able to soak up more oxygen and therefore the bacteria, in theory, is stronger and healthier. You've just got to remember that when you switch your filter off, make sure that the media doesn't dry out in the open air.

When using a sump any evaporation will actually occur in your sump filter, not your aquarium. So you must keep a check on the water level in the sump, especially during hot periods of weather.

A sump filter will swallow up an amazing amount of media, therefore choose larger grades of biological media. I would avoid media that come in very small particles, these will just make a mess and are really not suitable for sump filters. The type of biological media I would recommend would be open cell media such as Flocor, very porous media such as Alphagrog, and bio balls which are fabulous biological media. They are little bit smaller than ping-pong balls but are covered in little prongs, absolutely fabulous for bacteria to thrive and grow on.

You will notice in the diagram that I have placed a media grid underneath the media. Sometimes debris can collect at the very bottom underneath all the media, this doesn't normally happen with bio balls, although I have experienced it happening with Alphagrog. This media grid will help ensure there is always a void underneath the media so the water can flow freely between the two chambers.

Another bit of useful advice would be to use media bags inside your sump filter. This basically means that instead of having to scoop media out by the handful, the media is contained in a bag which just needs to be lifted out in one go. My philosophy is if you're going to go to the extent of installing one of these filters then do it properly and make life as easy as you can for yourself. Finally, if you really want to make your sump efficient when it comes to cleaning, you could put some outlets on it so that you basically attach a tube, open the taps and then drain all the crap out from the bottom.

A sump filter is definitely the way to go if you are going to set up a large aquarium containing lots of big fish. Commercially available canister filters will work, but they will become dirty very quickly, you will also need more than one filter if you have a big tank, these filters don't come cheap. For a fraction of the price of a large canister filter, you could set up a large sump filter that will completely blow any commercial filter out of the water when it comes to efficiency.

Choosing a Water Pump

The water pump will circulate the water around your system, therefore you need to choose the correct pump for your sump filter. Very much like any other filter that you install and the aquarium, the pump will need to circulate the water enough times to keep the water nice and clean and free from toxins. My 300 gallon aquarium has a couple of

running on it, I would highly recommend these pumps, you really don't get better for your buck. These pumps are very powerful so I actually only need one of them running, the other one is purely there for backup. People often say it's impossible to have too much filtration. However, it is on the other hand possible to have a pump that is too much for your aquarium/sump. For instance, one of my pumps will circulate over 3000 gallons of water an hour. If you have two pumps running then that 6000 gallons of water an hour. Even though things will work okay, the water will be running so quickly between the sump and the aquarium, there will be an absolutely terrific amount of noise caused by all the water running through the system. So bear this in mind when choosing your pump.

When choosing your pump, have a good look at the specifications that will be listed, they will look something like below:

Pump output: 2280 l/h 502 lmp. gal./h. 602 U.S. gal./h.
Delivery head: 3,10 m/wat.col. 10 ft. 2 in./wat.col.
Power consumption: 50 W
Hose connection (suction side): Ø28mm (1")
Hose connection (pressure side): Ø18mm (3/4")
Dimensions: 218 x 116 x 161 8.6 x 4.6 x 6.4 in.

Biological media for sump filters

The biggest advantage with sump filters over normal canister type filters is they can take an enormous amount of filtration, far more than you would ever be able to get into a canister filter. I would advise you to look at the type of media that people use in outdoor pond filters. This type of media tends to be a lot bigger in diameter, therefore one bag of bio balls, for instance, is a lot more efficient than a box of biological media that you would buy for a canister filter.

You can read more about different types of media for sump filters by clicking on the link below.

Read more about media for the sump filter

Nitrate Filter

A fact that not many people are aware of is a sump filter can often automatically transform itself into a nitrate filter, as well as functioning to remove ammonia and nitrite. I know this sounds hard to believe but let me explain how this happens. Your filtration system is not only there to remove dangerous toxins, it also removes fish poo, dirt, uneaten food and any other rubbish that you find in an aquarium. All this nasty stuff eventually turns into a sludge that settles in the bottom of your filter. A lot of this sludge will collect in areas of the filter that receives no water movement when this happens anaerobic bacteria grow in the sludge as they do not require oxygen to survive. Remember that the opposite of anaerobic is aerobic. Aerobic organisms such as nitrifying bacteria are responsible for removing ammonia and nitrite, but these need oxygen to survive. Now, this is the really clever part, the particular anaerobic bacteria that are living in all this sludge consume nitrate, so actually, all this horrible gooey mess that collects in your filter is actually working as your friend. Now obviously there is a fine balance between how much sludge you leave in your filter, and when you have the clean it. Actually, it's not really something you have to worry about, most of this sludge that collects won't be affected by filter cleaning as it will find its way into dead areas in your sump, so you can still clean the filter without worrying about completely removing all your anaerobic bacteria. To give you an idea of just how efficient a large sump filter can be, take a look at my 75-gallon sump filter which services my 300-gallon aquarium that contains four large adult Oscars. I cleaned my sump for the first time in a year a few days ago. Don't get me wrong, I have cleaned the mechanical filter sponges quite a few times as these do get clogged up, but I haven't actually emptied the sump completely. I can hear people saying "OH MY GOD, your water must be absolutely disgusting" well, it's quite the opposite, my nitrate levels are always low, even if I leave the tank for two or three weeks without carrying out a water change. This is purely down to the fact that my sump filter is now also functioning as a partial nitrate filter.

It's worth mentioning that these natural phenomena will only happen in a sump filter, it will not occur in a canister filter, hob filter, under gravel or any other filter apart from a sump. The reason being is the time sludge builds up inside these filters, water flow is completely restricted and the filter stops functioning. Because there is always a gap underneath your media in a sump, the sludge can build up quite happily but will almost never create a problem with restricted water flow.