Last Updated on November 19, 2022 by Matt

The “water column” is a phase used often in talking about fish tanks. 

It is used in most of our articles on this website, and indeed it is used very often on most aquatic websites.

But what is actually meant when the term water column is used? 

Most of the results if you do an internet search are results about the ocean. This isn’t surprising given the amount of research that is done!  It is also very interesting how sea temperatures change with depth.

But hopefully this article will clear things up a bit on what the term water column means in relation to an aquarium.

What is the water column?

Aquarium Water Column Layers

Well to put it simply, the water column is a conceptual description which refers to the entire body of water; from the very top right to the substrate. 

So referring to the water column just means all of the water in your aquarium. 

In general the water column is only the only in the actual tank at that time, not any residual water in the aquarium filter, sump, or any tubing. 

Now in the ocean, the water column is divided into many small sections of water so it is easier to understand. These all have very fancy names such as epipelagic and abyssopelagic. But don’t worry, the aquarium water column is just split into the top, middle, and bottom. 

A lot easier to remember and understand!

There is no exact science for exactly where the top, middle, and bottom layers start and end. It’s basically just dividing your tank into thirds vertically. 

Where is the phase “water column” used?

The phrase is normally used by more advanced or more experienced aquarists or hobbyists, as it is the correct and “proper” way of referring to your tank’s water.

In general more beginner aquarists simply say the aquarium water. It means the same thing! 

It is slightly clearer if you refer to your tank’s water as the water column, as there will be no confusion if you are asking for advice from a store or online in a forum. 

The term is also often used when referring to where fish prefer to spend their time. 

For instance guppies and molly fish have upturned mouths used for surface feeding. Therefore they prefer to spend time in the top level of the water column. 

Scavengers such as catfish, plecos, clown loaches, and kuhli loaches like to spend the majority of their time at the bottom of the water column. Here they can find fallen food amongst the substrate. 

Then there are the fish such as goldfish and angelfish who prefer to spend their time in the middle of the water column. 

The term can also have relevance to aquarium plants as well. 

There are the rooted plants which have to be planted in the substrate at the bottom of the water column. Then there are the floating aquarium plants, such as dwarf water lettuce, which are to be floated at the top of the water column.

It is also used when referring to a build up of algae or harmful substances such as ammonia or nitrate

For instance you will often read something in online forums like:

There has been an outbreak of algae in the water column of my tank, what do I do?

This just means that they are having trouble with algae in the tank.


Hopefully this short article has cleared things up a little bit. Don’t worry if you needed this article, unless you’re told you won’t know!

Essentially the term water column just refers to all the water in your tank.

Thanks for reading! 

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Coral Realm

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