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Which auto top-off pump is right for me?

One question we frequently receive concerns which of our two choices of
automatic top-off pumps is best.  Of course, the short answer is, "it
depends". We have two styles of pump available, a peristaltic pump and a
diaphragm pump.  The peristaltic pump works by means of a motor slowly
turning a rolling mechanism which pushes water through a section of
tubing by pinching the tubing flat with a roller, and pushing that bit
of water along until the next roller in the assembly contacts the tubing
again, etc.  The result is a slow but very consistent flow rate.  The
diaphragm pump works differently.  It is very similar to an air pump,
where a pair of flat rubber disks called diaphragms are actuated one at
a time, pulling water (or air) in on one side, then pushing out the
other side as the diaphragm closes.  Similar to how a heart pumps blood.
The main difference between the two for top-off purposes concerns the
ability of the peristaltic pump to prevent backflow when the pump is
powered off.  In some cases, a top-off reservoir may be situated so that
it is higher than the sump that it fills.  In this case, if a pump like
the diaphragm pump draws water from the reservoir, pushes it through
tubing to the sump, and then turns off, the water will continue to
siphon "downhill", potentially leading to overfilling of the sump.  This
is because the diaphragms are partially open when the pump is off.
Conversely, the peristaltic pump's roller mechanism simply stops in
place, and one roller is always in contact with the tubing, pinching it
closed.  Therefore, no water can pass while the pump is off.  A
peristaltic pump is the best choice when there is potential for unwanted

There are a few other considerations between the pumps.  The diaphragm
pump can move water faster than the peristaltic pump, about 20x as fast.
This may be useful in some situations, such as topping off a very large
system that our peristaltic pump, at 9 gallons per day max, cannot keep
up with.  The slow drip rate of the peristaltic pump makes it an
excellent choice for feeding fresh water through a kalk stirrer before
it enters the sump.  The diaphragm pump can also be used for this, but
you'll need to use an included micro valve to slow down the pump's
output.  This may impact the life of the pump a small amount, but it
isn't typically a problem in our experience.  The diaphragm pump is also
noisier, but not dramatically so.  Since it doesn't need to run very
long, typically only a minute or two, most people don't mind the low
buzz that comes from the pump at run time.  The peristaltic pump is
virtually silent, where you need to put your ear to the box in order to
hear it run.  Although it usually runs longer due to the low flow rate.
Both pumps only use about 2 watts of power.  Each pump is capable of
pushing or pulling water long distances both horizontally and
vertically, but the diaphragm pump can do several times more than the
peristaltic pump.  The peristaltic pump can handle at least 20 feet of
vertical head pressure though, which is typically plenty for most
basement/main floor split systems.

Lastly, the cost is a factor to be considered.  The peristaltic pump
costs a little more than twice as much as the diaphragm pump.  For many
beginners with a top-off reservoir located lower than their sump, the
diaphragm pump is all you would need.  Even then, there are ways to
prevent siphoning; see diaphragm pump instruction manual.  The
peristaltic pump is still the more reliable pump in terms of flow rate
(again, useful with a kalk stirrer), with a longer lifespan and easily
replaceable parts.  Silence is always a premium in the aquarium world,
so if you want a truly quiet system, you'll want the peristaltic pump.