I'm assuming it's measuring in µS/cm ..
I can't tell you what a "safe" range is for your fish, but I can tell you that it seems logical to me that as the water gets impurities over time (nitrates, waste, minerals from rocks, etc) the conductivity will creep up. It should go back down when you do your routine water changes, so what I would do is see how high it goes before your routine water change. Do this a couple of times over the course of a few changes so you can get some sense of where the EC will routinely be when the tank is in need of maintenance, then set the alarm slightly above that level. That way the normal operating parameters of the tank won't trigger the alarm, unless you get behind on maintenance or something has gone wrong.
I can't tell you what a "safe" range is for your fish, but I can tell you that it seems logical to me that as the water gets impurities over time (nitrates, waste, minerals from rocks, etc) the conductivity will creep up. It should go back down when you do your routine water changes, so what I would do is see how high it goes before your routine water change. Do this a couple of times over the course of a few changes so you can get some sense of where the EC will routinely be when the tank is in need of maintenance, then set the alarm slightly above that level. That way the normal operating parameters of the tank won't trigger the alarm, unless you get behind on maintenance or something has gone wrong.