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>How does a sump work?
>Why does it not overfill when it's placed below the aquarium?
A sump is actually part of a two-part system (if you don't include the
pump). There is also an overflow / surface skimmer that goes in the tank.
Typically the overflow (overflow being the *device*, not a mess on the
floor ;-) is either part of the tank with a standpipe or is a "hang on
tank" siphon design. The standpipe is the simplest and more reliable
option, being just a pipe coming up through the bottom of the tank to a
level just slightly below the desired water level. The siphon unit is set
up so as to prevent the siphon from breaking should the sump's pump stop,
and is also set in the tank just below the desired water level. There is a
good picture of such a design in the lower right of this page on the krib:
The idea is that in normal use the sump pump is constantly emptying the
sump into the main tank (or "show tank"). The stand pipe or overflow is
constantly draining the show tank into the sump. This maintains a
"balance", with the level in the show tank being set by the height of the
stand pipe or the depth of the edge of the overflow siphon in the tank.
Should the pump fail, the water in the show tank drains down until the
overflow's edge stops water flow or the standpipe is slightly *above* the
desired water level.
The neat part is that all evaporative losses show up only in the sump, and
you can do your water changes and such without ever altering the water
levels or flows in the show tank. The problem is that should something
happen to the plumbing between the standpipe/overflow and the sump it *is*
possible for the sump to go dry and the resulting water to be pumped into
the show tank and then be dumped onto the floor (through the overflow or
standpipe) instead of back into the sump. Float valves can be used to shut
the sump pump off should something like this happen.
BTW, there are also automatic top-off systems that are sometimes used with
UNIX Systems Administrator