Granted…not a very sexy title for a blog post, mainly because the word “sump” doesn’t have a lot going for it, but I’m glad you are reading this anyway.  Why?  Because I believe there is a lot of confusion out there about what aquaponics sump tanks are, when you might need them, and how to size them.

The easiest way to explain when you might need an aquaponics sump tank in your system is to start with when you don’t need one.   You can skip the addition of a sump tank if the water volume in your fish tank is greater than or equal to the total volume of your empty grow beds.  Why?  Let’s say that both your fish tank and your grow beds in aggregate hold 100 gallons of water.  The media you use in your grow bed(s) will fill about 60% of their volume (they will be full to the top with media but there will be lots of space between the bits of media for water). This will leave only about 40% of the bed volume to fill with water on each flood cycle.  Forty percent of 100 gallons is forty gallons, so at most you will be removing forty gallons from the fish tank during the flood cycle, thereby leaving 60 gallons in the tank.  While the fish probably won’t appreciate the constant fluctuations in the size of their living arrangement, they can live with it.

Now let’s say that you add on another grow bed with the same 100 gallon volume.  Now, every time both grow beds simultaneously fill, you will have pulled 80 gallons of water from the fish tank leaving only 20!  Depending on the dimensions of your fish tank, your fish might be swimming on their sides and your pump might be screaming at you for lack of water.  Not a happy situation over all.

This is where an aquaponics sump tank comes in.  Its sole purpose is to enable the fish tank water height to remain constant regardless of how full or empty your grow beds are.  This becomes possible because the fluctuating water height shifts from the fish tank to the sump.

Broadly, a “sump” is an area where liquid run-off accumulates.  For example, water that seeps into a basement drains into a sump.  In a boat the sump is also known as the “bilge”.  In an aquaponics system the sump sits at a point lower than the grow beds and is the tank into which the grow beds drain.

In an aquaponics system with a sump, the water goes in and out of the grow beds via the sump tank, and the water level in the fish tank remains undisturbed.  With this design, the fish tank water is exchanged by having new water pumped in from the sump, which causes water from the fish tank to drain by gravity either back into the grow beds (CHOP or CHFT PST) or into the sump tank (CHOP2).  This allows you to add far more grow beds than you could otherwise, even going all the way up to a 1:3 ratio of fish tank to grow bed volume.  After that, the constraint on adding more beds is more about the amount of plant nutrients that your fish can safely provide versus how much water is flowing through the system.

How large of a sump tank do I need?

  • Since the sump tank is the source of the water that flows into the grow beds and receives all the water that drains from the grow beds, your sump should be big enough to handle the situation when all of your grow beds are simultaneously full.  Just add the total water volume of your grow beds, then subtract the displacement effect of the grow media (for Hydroton we’ve found that 55% of the water is displaced, and 62% with 3/4″ gravel).
  • Next, calculate the minimum amount of water that must remain in the sump tank to cover the pump and insure that it does not run dry and burn out. We use 3″, but your pump intake height may be different than ours. This volume should be added to the grow bed water volume calculated above. The total is the minimum volume you need for your sump tank.
  • By the way, you can easily connect two sump tanks with a simple bridge siphon and create essentially a single, larger sump tank.

What else do I need to consider?

To save floor space, pay close attention to the height and width of the sump tank to make sure that it will fit, unobstructed, under your grow beds.  If you have a dirt floor, you can sink the sump into the ground to make height less of a consideration.

And that is it!  Hopefully now you have a clear answer to the question “To Sump…or not to Sump?”