Freshwater Aquarium Sumps: An Ultimate Guide

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Sumps have been used to filter aquariums for decades. They have always been really popular amongst those fish keepers who have marine aquarium setups, but less popular in the freshwater fish keeping community. In this article, I look at sumps for freshwater aquariums and consider many of the pros and cons.

What Is An Aquarium Sump?

Let’s start by considering ’What is an aquarium sump?’ At a basic level, an aquarium sump is just a container to hold water where the filter media can process the aquarium waste. Water enters the sump, passes through the media, and is returned to the aquarium.

Aquarium sumps can essentially be made from anything, providing it holds water. Some aquarists are willing to literally spend thousands of dollars on a sump set up. In reality, you don’t need to spend much money at all.

An old aquarium can be used as a sump as can a plastic tote. I have visited fish rooms where a plastic garbage can has been used as a sump and I myself have used Really Useful Storage Boxes (like these ones from Amazon) as sumps.

A sump doesn’t have to be pretty, it just has to hold water and be large enough for whatever media and equipment you want to put in it.

How Does An Aquarium Sump Work?

Aquarium sumps can be as simple or complex as you want them to be. In their most basic form, aquarium sumps are like any other filter, just on a larger scale.

In the case of an under aquarium sump water drains down from the aquarium above, usually just via gravity, into the sump. It then passes through any number of chambers that contain either media or equipment before being returned to the main aquarium. The image below shows what a very basic sump may look like.

Sumps can have as many or as few chambers for the water to pass through as the aquarists wish.

With ’over aquarium sumps’, the water is usually pumped out of the aquarium, into the sump where it passes through the media chambers and returns to the aquarium via gravity.

Over aquarium sumps are usually used on really large aquariums like plywood aquariums where there is no way to place a sump under the aquarium.

What Are The Benefits Of An Aquarium Sump?

There are many benefits to using a sump on a freshwater aquarium setup. These include:

  • The sump increases the overall volume of water in the aquarium
  • Equipment such as heaters can be put in the sump rather than in the main display aquarium.
  • Sumps are highly customizable
  • Sumps are easy to service
  • The sump can provide a refuge for fish which need to be kept separate
  • Sumps can be used to store extra ‘cycled’ media for use in other tanks
  • Increased aeration of the water

Increased Water Volume

Adding a sump to your freshwater aquarium increases the overall water volume of your aquarium. The more water an aquarium has, the easier it is to keep the parameters in that water stable. If you place a 50-gallon sump under your 150-gallon aquarium, the overall water volume becomes 200 gallons.

By increasing the overall water volume, you increase the stability of that water. There are many fish in our hobby that can cope with less than perfect water conditions, providing those conditions are stable.

Hiding Equipment

Let’s be honest, no one wants to see fish-keeping equipment in their aquariums. Having a sump under our aquariums gives us somewhere to place that equipment.

I have used my sumps as a place to put heaters, airstones, CO2 diffusers, and even algae scrubbers. Anything we can get out of our aquariums makes more space for fish and a more enjoyable viewing experience when we look into our tanks.

Placing equipment in the sump rather than the aquarium has a couple of additional benefits. It can be easier to service equipment or change heaters when they are in the sump, especially in a heavily aquascaped aquarium.

If you keep boisterous fish like Oscars or other large cichlids, heaters and other equipment are easily damaged. If it is in the sump, the fish can’t crash into it and break it.

Sumps are highly customizable

The reason I love using sumps so much is I can customize my filtration to an individual aquarium’s needs. I have a sump running at the moment which uses sponges, K1 media, gravel, filter floss, and carbon to filter the water. I have taken my time to set it up exactly how I want it and I know there is the option to change it around in the future if my needs change.

Easy to Service

Being easy to service is not something to be overlooked. I can clean or swap the sponges in my sumps quicker than I can in my canister filters. The same is true when swapping bags of carbon or removing limescale build-up around the heaters or pumps. Because sumps are so easy to service, most of us are more likely to actually service them.

A place of refuge

Occasionally a fish may need to have its own space. Maybe it has been attacked by another fish and needs to recuperate or maybe it was the one doing the attacking and a spell in confinement might do everyone some good.

Either way, a chamber in the sump can be used as a refugium where the fish can be placed alone, yet doesn’t have to live in a different aquarium.

A refugium can also be useful if your fish spawn without your knowledge and you need somewhere to place the fry until you can find homes for them.

Storage of ‘cycled’ media

By keeping a bag or two of bio rings or an extra sponge filter in the sump, you will always be ready if you need to set up another aquarium at short notice. You can easily remove the media from your sump and place it into the filter of the new aquarium and you are ready to go. No need to wait for the new aquarium to cycle.

Increased Aeration of the water

Almost all fish like to live in highly oxygenated water. Many aquarists however don’t want to see a stream of bubbles from an airstone in their aquarium. The answer? Place an airstone in the sump just before the return pump.

When the pump sends the water back to the main aquarium it will be full of dissolved oxygen. The result will be highly oxygenated water being returned to your aquarium.

How Large Should An Aquarium Sump Be?

There are no hard and fast rules about how large a freshwater aquarium sump should be. Needless to say, the bigger the better, but that isn’t especially helpful.

As a rule of thumb, aim for your sump to be at least 20% the volume of your main aquarium. As an example, for a 100-gallon aquarium, you would want a sump of at least 20 gallons.

When deciding the size of your sump, there is one major factor many newcomers fail to take into account. If your power goes out, some water from the display aquarium, plus whatever is in the pipework, will drain into the sump. If you haven’t allowed for this, the first time your power goes out you will have water on the floor!

Aquarium SizeSuggested Sump Size
100 Gallons20 Gallons
125 Gallons25 Gallons
150 Gallons30 Gallons
180 Gallons36 Gallons
200 Gallons40 Gallons
250 Gallons50 Gallons
300 Gallons60 Gallons

What Media Should Go In An Aquarium Sump?

The answer to ’What filter media should go in a sump?’ is extremely easy, anything you want. You can use sponges, bio-balls, ceramic rings, gravel, old socks, or just about anything else.

Filter media is just a place for bacteria to live. With one or two exceptions, bacteria will colonize just about any surface.

There used to be an aquarium shop in Seattle that literally used dirty gym socks as their filter media. The bacteria didn’t care and neither did the fish!

What Order Should The Aquarium Sump Media Be In?

What order should the filter media go into a sump is something which is widely misunderstood, even by experienced fish keepers. No matter what filter you are suing, the media should ALWAYS follow the same format;

In simple terms, this means the water should first be filtered mechanically to remove any solid items from the water. The sponge is the most widely used media for mechanical filtration. Always place your sponges from most coarse to finest.

That way the largest items are removed first, then the medium-sized bits of waste, followed by the finest particles of waste. Placing the sponges the other way around will lead to the fine sponge becoming clogged prematurely.

Once the water in the sump has passed through the mechanical filtration it should next go through the biological stage. In a sump, this is usually bio-balls or ceramic rings or something similar.

Biological filtration is when good bacteria convert the fish waste from ammonia to nitrite, then from nitrite to nitrate. It is important the water spends as much time passing through this stage of the filtration process as possible.

If the way is traveling too quickly the bacteria won’t have a chance to do its job.

The final stage of normal aquarium filtration is the chemical stage. This usually means the water passes through activated carbon or another product that will chemically change the water.

Chemical filtration is too big of a topic for this article, but one key factor is that the water should be physically as clean as possible before the chemical stage of filtration, which is why it is always the final stage before the water is returned to the display aquarium.

Should An Aquarium Sump Be Under Or Over The Aquarium?

It is perfectly acceptable to place the sump either under or over the aquarium. Usually, with smaller aquariums, the sump is placed under the aquarium. When using sumps on aquariums that are over 1000 gallons, the sump will normally be placed above, or ‘overhead’.

There is no right or wrong place to put the sump. The decision will be based on the circumstances around the individual aquarium.

Are There Disadvantages To Using Aquarium Sumps?

There are some well-known disadvantages to using sumps to filter aquariums. The first major disadvantage is cost. Typically, an ‘off the shelf’ sump will cost 2 to 3 times the price of a canister filter for the same size aquarium.

However, the beauty of using a sump is that a clean plastic tote from Home Depot can be made into just as effective a sump, for a fraction of the cost.

Another big issue with sumps is that when things go wrong, they can lead to your home being flooded. There is a far higher chance of an incorrectly set up sump flooding your house than any other type of filter.

Precautions should be taken, especially thinking toward potential power failures. You don’t want your entire display aquarium to empty into the sump if the return pump was to go off during a power outage.

My Final Thoughts On Sumps

I am a massive fan of sumps. I am a real filter geek and being able to customize my filtration to be set up exactly how I want it for an individual aquarium really floats my boat. If your aquarium setup allows for a sump, then I would seriously consider one.

Sumps offer you the ability to spend as much, or as little as you want to on your filtration and they allow for your filter to adapt as your aquarium changes.

About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James

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