Sump 101
The American Heritage® Dictionary of the English Language, Fourth Edition defines a
sump as:
A low-lying place, such as a pit, that receives drainage.
This is an accurate, albeit simplistic description of a sump. Let’s tailor the definition a bit
to be more descriptive of our usage of the word in the hobby:
A tank or container placed lower than, and receives drainage from, the display
aquarium.
The obvious first question most beginners have is why in the world would I want one of
these? They add more complexity and possibilities for disaster, don’t they? Well, if
installed correctly, the benefits to installing a sump on your aquarium far outweigh any
downsides, of which there are really only a few. The benefits include increasing total
water volume, providing a place for equipment and filtration, keeping the surface of the
water free of contaminates, aerating the water, simplifying water changes, and keeping
the water level stable in the display tank.
Let’s discuss the last point first. An annoying feature of having a tank full of water in
your house is that water evaporates, lowering the water level in your tank. Depending on
the size of your tank and the humidity of your house, water levels can easily drop as
much as an inch in one day. This can be aesthetically annoying as well as a maintenance
headache. A sump is plumbed into your system in such a way that the water level in the
tank remains constant while the water level in the sump lowers with evaporation and
raises with the addition of fresh water.
Increasing the total water volume in a saltwater aquarium is desirable because the more
water you have in your system, the more stable the system. Water temperature and
chemistry changes more slowly when you have more volume, which makes it easier to
keep the system within optimal parameters. For example, two gallons of evaporation
from a 20-gallon aquarium would raise the specific gravity from 1.026 to 1.029, while
that same two gallons evaporated from the same 20-gallon tank with a 10-gallon sump
attached would only raise to 1.028, resulting in a much more stable salinity for your
aquariums inhabitants. As the old saying goes “The solution to pollution is dilution,” and
the more water you have, the more dilution you have.
Having a sump on your system allows you to hide equipment such as heaters, filters and
skimmers so that they don’t disturb the aesthetics of the display tank. Also, with a sump,
it is not necessary to keep powerheads in your tank for circulation. Of course, you must
still maintain the proper amount of circulation. I’ll go into this in detail in Part II of the
article.
<<more sumps with equipment>>
An integral part of a sump is the “overflow”. The overflow is the mechanism that drains
water into the sump from the display tank. The overflow resides in or “hangs on” the
display tank in a fixed position so that any water that rises about that level in the tank
“overflows” through the drain into the sump. This is how the water level remains
constant in the display tank. A wonderful side effect of using an overflow to drain water
into a sump is that the surface of the water is constantly skimmed, removing surface film
from the tank. This improves gas exchange that can boost oxygen levels in your tank and
lower CO2 levels. Also, not having the surface of your tank look like pond scum is a
good thing, don’t you agree?
<<picture of skimming at overflow>>
Additionally, overall aeration of the water is improved by having the water fall over the
overflow, down the drain and splash into the sump. This assures good gas exchange
between the water and air.
Sumps can also help make water changes a little easier. You can remove the water in the
sump without lowering the water in the display tank. Also, you don’t have to lift the
replacement water up over the rim of the aquarium.
A sump is also an excellent place to dose chemicals or replace make-up water. This
allows whatever you are adding to the water to mix with the current water before it is
makes it’s way to your prize fish and / or coral.
There are many different things that can be used as a sump, and for every type of sump
there are a dozen ways to use them. The basic containers commonly used for sumps are
small aquariums, custom acrylic sumps, small plastic containers, and large plastic
“troughs”.
<<pictures of different types of sumps>>
Sumps can also serve other purposes than hiding equipment and adding water volume.
Many people design their sumps to be used as a trickle filter, algal filter, or refugium as
well as simply holding water and equipment.
<<pics of trickle filter, algal filter and refugium built into sump>>
The plumbing for a sump is actually rather simple, once you understand the concept. A
mandatory feature is that the sump must be lower than the waterline in the display tank.
Because of this necessity, most people install their sumps under the tank in the stand.
Other possibilities for installation locations are: beside the tank, or in extreme cases,
some build “fish rooms” in their basements and locate their sump there. It is important to
keep in mind however, that the lower you locate your sump the more difficult it is to
return the water back to the tank as you’ll have to lift the water higher.
We’ve already discussed the overflow, which is the mechanism for removing water from
the main tank, but we need to look at them in more detail.
In most cases, the ideal overflow is built into the tank utilizing a hole “drilled” into the
tank inside the overflow. Aquariums with this feature are sometimes called “pre-drilled”
or “reef-ready” tanks. If you are planning a new tank and haven’t purchased it yet, I
recommend you seriously consider purchasing a pre-drilled tank. It is possible to have an
existing non-drilled tank drilled to your specifications as well, which is a good alternative
for those that already own a non-drilled tank, or want to have the holes drilled in
“custom” locations.
This type of overflow is quite simple and has few opportunities for failure. The overflow
is designed to allow water to spill “over” it a few inches below the tanks rim. Because
water will only spill into the overflow if the water level is above the top of the overflow,
the water level in the tank will remain constant. Also, no water can drain from the tank
after the water level lowers to the top of the overflow. This type of overflow can fail if
the drain hole becomes clogged, preventing water from draining into the sump, although
it’s very easy to prevent this from happening, and is a subject I will go into detail about in
Part II of the article. About the only downside to a drilled tank is that the overflow will
take up some amount of room in the tank reducing the amount of space you have usable.
<<pictures of reef-ready overflows>>
A reef-ready, or drilled tank, doesn’t help much for someone that already has a tank up
and running and doesn’t want to break it down to have it drilled. For this situation hangon overflows were invented. They hang on the back of the aquarium and over into the
aquarium. Hang on overflows are less reliable and can handle less water flow than a
“drilled” overflow. This is because overflows must fight against gravity and flow the
water “over” the top of the tank rather than through a hole in the tank. Hang on overflows
use a technique called “siphoning” to move the water up and over the edge of the tank
and down into the sump. As long as the siphon is maintained, there is no problem, but if
the siphon is broken for any reason, then water will cease to flow into the sump and the
display tank will continue to fill up until either the sump is drained or water flows over
the top of the display tank.
Click the link for an animated description of how hang-on overflows work:
http://www.saltyzoo.com/other/overflow-dynamic-labeled.gif
<<skipper, I think the animated gif would work better as a slideshow. Is that possible to
integrate into the article?>>
<<more pics of hang-on overflows>>
From the overflow, the water flows through the standpipe down the drain line into the
sump. From there, the return pump (named such because it “returns” the water to the
tank) pushes the water back up into the display tank. This raises the water level which in
turn causes water to spill over the top of the overflow and start the whole cycle over
again.
There are very few reasons why you would not want a sump. Really the only reason I feel
is valid is if you have no space to install one. For several years I ran a 10-gallon display
tank without a sump. The tank sat on a dresser in a small bedroom, so for obvious reasons
a sump was not feasible. As that tank proved, a sump is not required for a healthy,
beautiful tank, despite all the benefits a sump provides. Coincidentally, that 10-gallon
tank is now being used as a sump for a 30-gallon display tank.
I hope this article has given you a better understanding of what a sump is and the
mechanics behind it. In Part II, I will cover methods to:
 Determine the proper size and material for your sump
 Determine the proper drain hole size and placement
 Determine the proper return pump for your sump
 Reduce risk of flooding
 Reduce bubbles in display tank
 Reduce noise associated with overflows and sumps