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RE: New tank set-up- questions, etc.


The prefilter/skimmer-box part of a wet-dry filter system is the place in
the system you are asking about wether a siphon break can cause the
aquarium to overflow. The answer is yes if the following conditions are
met: (1) There is some obstruction in the path of the water draining from
the tank to the sump (such as an air lock or loss of siphon) and (2) The
aquarium does not have enough space, or "headroom", left to contain the
(now limited) amount of water being pumped from the sump into the aquarium
until the pump runs dry.

The equilibrium siphon system:

There are basically three pieces to this part of the filtration system: The
skimmer box, the prefilter, and the siphon tube. These work together to
form an equilibrium siphon system. Its a very simple system. To understand
how it works, imagine two glasses of water sitting on your kitchen table.
You take a short piece of airline tubing, fill it with water, and put one
in one glass and the other end in the other glass. You now have two glasses
of water connected by a tube full of water. Now, what will happen if you
add water to one of the glasses? Water will go through the tube until both
glasses have the same level of water. They seek equilibrium. This is how it
works on the aquarium as well.

The prefilter and the skimmer box are usually physically connected together
by a plastic screw but the water pathway between the two is by equilibrium
siphon. The equilibrium siphon is usually a clear  plastic "U" shaped tube
called  (what else?) a U-tube. This tube is inverted with one end in the
prefilter and the other end in the skimmer box, both of which are full of
water, and is then itself filled with water by inserting a piece of airline
up into the highest part of the tube, which is the bottom of the "U", and
sucking the air out of it. The important thing to remember is this tube
must always be completely full of water. The prefilter and skimmer box are
designed to keep it this way even if the electricity goes out and
everything shuts down. The only thing that can cause a siphon break is if
someone lifts the tube up and dumps the water out of it or, more likely,
air bubbles get trapped in it and eventually the air pocket gets large
enough that it causes an air lock. Its a good idea to make a quick visual
inspection of the tube on a daily basis. I usually do it when I'm feeding
the fish. Air bubbles can be created by water spilling over the gate into
the skimmer box and these get sucked into the U-tube where they collect. I
should mention that some manufacturers don't use U-tubes exactly but their
products rely on exactly the same principle and are subject to the same
problem. The skimmer box usually has a variable height adjustment. This
allows you to set the headroom in your aquarium to a level which will
accommodate the water volume of the sump if the above mentioned situation
occurs and the water stops draining from the aquarium.

Preventing overflows: 

In my experience, there are two ways to set up an aquarium that uses a
wet-dry style of filtration so that it won't overflow either the aquarium
or the sump. One is to consider the possible conditions that would cause an
overflow situation and then carefully compute the volumes involved. This
information would then be used to determine solutions but the solutions
would only work for that set of measurements. This method is based solely
on mathematics and you better get the measurements right. It may be the
only alternative in some cases. If you are not very familiar with these
filtration systems then the method I would suggest is to set up the system
in a place where some spilled water is not going to be a problem and test
it by trial and error. It may seem like a lot of extra work but it will
teach you a lot about how these systems work and the relationship of the
variables involved.

The first wet-dry filtration system I had experience with was an Amiracle
and I must say that their one page instruction sheet vaguely described how
to set it up at best and did nothing to explain what can happen if you use
it on a larger aquarium or what size pump to use or how to adjust the
skimmer box or anything else important. The only overflow situation I was
aware of was the sump overflow caused by water back-siphoning down the
return line. Even this (the most common type of overflow situation) was not
explained on the instruction sheet. I learned that one from the very good
LFS I bought it from. Fortunately, I installed it on an aquarium that was
set up on a concrete floor and was near a floor drain so the unexpected
overflows weren't a big problem. I learned the hard way (although not as
hard as if this had been a saltwater tank in a nicely carpeted office) that
there can be different causes for overflows.

You said you were planning on setting up a 20 gallon aquarium so, my advice
to you is to set it up where you can test it. Fill it up and plug it in.
Then, when it appears to be running normally (give it a few minutes to
stabilize water levels) with the proper amount of water in the sump, unplug
it and see what happens. This test can be used to mark the location of the
small hole you're going to put in the return line. Its OK to put this hole
below the normal (while the system is running) water line of the aquarium
(for CO2 conservation) but you must put it above the point (indicated by
the water level in the aquarium) at which the sump overflows. Another test
you can run is the aquarium overflow test (as you were concerned about in
your post). This test involves setting the height of the skimmer box
correctly. First get the system running normally and give it a few minutes
for the water levels in the aquarium and the sump to stabilize. Then pull
out the U-tube and see if your pump runs dry before the aquarium overflows.
If there is a problem you can either lower the skimmer box height to create
more headroom in the aquarium or raise the height of the pump (or pump
intake) so that it pumps a smaller volume of water until it runs dry.
Basically, you need to run tests to determine the minimum and maximum water
levels in the sump, the height of the pump in the sump, the height of the
skimmer box in the aquarium, and the additional water capacity the aquarium
can handle when the contents of the sump are pumped into it. When in use,
these types of filtration systems keep a set water level in the aquarium
which is determined by the height of the skimmer box. Changes in water
level occur in the sump. Changes in the flow rate, either by changing the
pump size significantly or restricting the water flow (such as a blockage,
or an air lock or loss of siphon in the U-tube) can and will alter the
respective water levels of the containers (the aquarium and the sump).
Changing the aquarium size with respect to the sump size will also affect
the capacities of both with respect to the conditions of overflow discussed
above. As you can see, trying to explain it in words is a bit complicated.
I suggest you run some practical tests to get some experience with it
before you set it up in an office.  I use plastic electrical tape to mark
the minimum and maximum water levels on the sump after I've run the tests
and adjusted the hardware.
One feature of the Amiracle system I have (an SL-150) that I don't like is
the width of the slots in the skimmer box. I think they are too wide and
allow small fish such as Otocinclus to get through. I have another brand of
skimmer box that uses narrower slots and I like it much better. I hope this

--- Eric