"Wayne Sallee" wrote in message ...
He could use two float switches :-)

YES, the top switch would shut down the pump
in case of overflow. I have already said that...
Evenry device can fail - instead of spending
big bucks on the "more reliable" I would rather
invest in an additional protection device agains
of the failure of the main one.

I use valves and control the drip rate rather than float
switches, but if I were to use a float switch, I would
tend to lean to a nonmechanical because in reef tanks,
there's a lot of growth that likes to get in to things.

Growth is limited in unlighted sump, and growth can
be managed with periodic maintenance... If you clean
the switch every month in vinegear it will be ok...
Also, the second switch working as the overfill shut-off
is normally not submerged, so there is no growth there.

But a nonmechanical switch could be less reliable than a
mechanical - depends on the design.

That is true. Mechanical switch usually controls the pump
directly or using simple relay. Non-mechanical solution usually has
a complicated controller board with a lot of electronics, software.
Read: it has A LOT OF VARIABLES which can go wrong.... :-)
In effect, non-mechanical solution can be less reliable!
Against intuition :-)

Also anyone looking at the possible errors of such things
should always look at the worst case senerio. Like will it
stick on, or stick off, how many gallons will go in, and such.

That is why I have my mechanical switch on a timer...
The timer is set to turn the system once per day, for 5 minutes.
During 5 minutes there is not much water pumped throught
thin tubing, so even if the switch will fail, it will not be a disaster
like in the event of whole bucket of freshwater dumped into the sump
and maybe overfilling sump and flooding the wooden floor :-)
Another protection is the second switch on top of the sump.
It will shut the pump in the event of overfilling.

Like Wayne said - it is not enought to install the best switch to
be safe... You need to design your top-off system and think
of any scenario which can happen - power outage, pump
failure, switch failure, overfilling etc... and check if you are
protected against all the likely scenarios.