"Jim Reynolds" wrote in message news:o2xjb.792901$uu5.139648@sccrnsc04...
Can someone explain how a top off system works or point me to a site that
explains the mechanics of the system ?

Thanks,
Jim Reynolds

The other posters have explained it pretty succinctly.

The only thing I have to add is a suggestion of how to implement one.

I had a reef tank for about 3 years, and it did *really* well-inverts
spawning, etc. To top it off, I purchased a float valve (all plastic,
no metal at all) from a plumbing supply. I teed off of my RO filter,
and ran the hose to the sump.

Dead level all the time. Salinity never varied, never leaked or
overfilled for 3 years.

Cost a total of about$10.00, including the reducing fittings and hose.