Making a Freshwater Aquarium Sump

Posted on July 12, 2011



It is said time and time again in aquarium literature that larger aquariums are better for fish and easier to look after due to the extra buffering capacity offered by a larger volume of water. Often though, most aquarists start off with smaller tanks which can quickly become overcrowded if livebearing fish are kept, or if certain specimens start to overgrow their once roomy tanks.

Once method of increasing tank water volume without actually increasing the size of the tank is by using a “Sump”. Sump tanks are mostly used in marine aquariums where water is siphoned from the main tank into another tank below. The water in this tank can be mechanically filtered by medium or biologically filtered by using aquatic plants before being pumped back up to the bank tank using a powerhead. As well as being a excellent filtration method, sump tanks also increase the overall volume of water, which offers some of the benefits of a larger tank such as more stocking/buffering capacity and easier maintenance.

This diagram provides a simple overview of how a sump tank works. In reality however, it is slightly more complex than this, with valves, bell siphons and slits to prevent flooding should the pump stop working or the siphon is air-locked.

Sump Tank

A typical Sump tank

For owners of smaller aquariums or those who don’t want the complexities involved with a sump tank, there is another simpler option.

The Passive Sump tank.

Essentially, a passive sump requires little/no power to operate and there is no risk of flooding associated with a typical sump tank. It could also be used to decorative effect or even to grow food! (More on this later)

This diagram shows how a passive sump works, and unlike the sump system above, this system is actually that simple! The two way siphon joins the two bodies of water together and allows dissolved minerals etc to diffuse from one tank into another. For example, ammonia diffuses from the main tank into the sump tank where ammonia concentration is lower due to the plants. Of course, simple diffusion is a slow process so the sump tank should be used in conjunction with an active filter system and not treated as a replacement for true filtration. One situation where passive sump would be useful is in Natural Planted Tanks (NPT) where the number of powerheads etc are kept to a minimum and fast growing plants such as Elodea densa or Hornwort (which are not regarded as particularly attractive) can be grown to curb algae growth.

Passive Sump

Passive Sump tank

Passive sump systems should not be used as an alternative for true filtration in most cases, however, one of the benefits of a sump tank in marine systems is that filters and powerheads can be hidden from the main tank. This is whee the active sump tank comes in.

Active Sump

This diagram shows how an active sump system works.

Active Sump

Active Sump Tank

Very similar to a marine sump (first diagram), however the sump tank itself is placed besides the main tank like the passive sump system. Water is pumped out of a sump tank into the main tank using a powerhead, which causes the water level in the main tank to rise. The difference in water level causes water to be siphoned from the main tank bank into the sump tank to restore the difference. Because the water level of the sump and the main tank are similar, the is less risk of flooding should something go wrong (if the pump stops working, the siphon simply stops moving water) and therefore fewer valves/bell siphons etc required.


Remember how I wrote that food can be grown using this system? Aquaponics (a fancy way of saying “growing plants using water alone”) essentially relies on a sump system to deliver minerals excreted from fish to plants.

The diagram below shows the gist of it.


Aquaponics System

I have grown herbs such as Rocket this way with success and many vegetables seem to grow better in water/nutrient solution alone rather than soil. The grow bed does not contain any soil, only gravel and clay rocks which act as a support for the plant roots. The gravel is large enough for water to freely permeate and deliver minerals to the plants root system.

My Sump system (will Update!)


Currently a passive sump system so far. I used it to overcome the limitations of my non-glass tank. Because the tank is opaque, the plants inside only receive natural light from above unlike glass tanks which receive light from all sides. This means that the plants (which I rely on for biological filtration and nitrate removal) aren’t as efficient as they could be. The sump tank (or two 1 litre glass bottles in this case) are used to grow Elodea quickly to remove nitrates faster, and the Elodea in the bottles grow considerably quicker than those in the dimmer main tank and much faster than if they were isolated from the main tank (presumably because of the higher mineral content in the main tank water).


The water is kept circulated between the main tank and the sump by raising the sump periodically, causing water to siphon from the sump to the tank. Once 1/4 of the water has been removed from the sump, it is placed back down to normal level and water begins to fill the sump to normal level again. It’s not an ideal system, but so far without access to the powerhead outlet, it works.


I am planning to convert this sump system to active by using some of the output from the powerhead to grow the Elodea faster, but the powerhead is currently underneath thick growths of plants and roots which I don’t want to disturb, so we’ll see how it goes :)

I might try to grow a few edible plants by making another shallower sump container and filling it with gravel and other goodies such as worms.

…and now we wait…