Probably a temporary drop of a few points in S.G. won't hurt much, although snails will take a beating. We forget that reef salinity near the surface will frequently drop temporarily due to heavy rain fall; at least that's what I've read. I think long term exposure to salinity outside of normal reef parameters is much more harmful.

There was a thread, about 50 pages long I think, on reefcentral named "DIY auto top-off with solenoid" or something like that. I made an auto top-off somewhat based on that thread and it never failed in over a year of service. The trick is to have some redundancy, like a solenoid that is opened by a float switch, and another float switch/solenoid higher in the sump to close the system in the event of failure on the first one. The primary solenoid can also be controlled by a decent timer (that's what I did) and the float switch only activated occasionally; mostly it stays out of the water. This helps keep the "snail taking a nap on the switch" syndrome to a minimum. BTW, there are float switches and then there are float switches; I think someone's got to be making decent ones. That thread has TONS of info about the whole concept of auto top-off. Or, you could buy the tunze unit that so many people are nuts about; just eat peanut butter sandwiches for a while!