Samwyse wrote:
Well, I spent the past week with relatives about 200 miles from here, so
the tank had plenty of time to "settle down". (For those joining late,
the tank contains live rock & sand, and one mantis shrimp; no little
vertebrates yet.) The first thing I noticed when I arrived home was a
slurping sound. Sure enough, the sump was about three gallons low,
which I replentished by pouring fresh RO water into the sump.

Holy cow! I just dodged a bullet!

I checked everything last night just before going to bed, and it all
seemed fine except the small air bubble in the syphon had grown a lot
bigger while I was gone. This morning, I was resetting the timer for
the lamps (from 12 hrs/day to 9) and decided to look at the SG before
going to work. There was something odd about the way it was floating,
but it took me a moment to realize that the water level in the tank was
actually touching the glass tank covers! My wife and kids are still at
the in-laws, so if I'd gone to work without checking I'd have come home
to about ten gallons of water on the floor!

I killed the power to everything and about an inch of water syphoned
out. Judging from the water level in the sump now, I suspect that most
of that three gallons that I thought I was missing had just moved up the
hill. Looking back, I had thought that three gallons seemed like a lot
to lose to evaporation. Plus, I noticed that the floating hydrometer
was just touching the plastic lid, but I'd assumed that it was due to
the increased salt concentration. I'll know better next time.

The "fabric" in the filter looks pretty dirty, so I guess it was
restricting the water flow just enough to cause trouble. I'm going to
leave everything powered down today, but tonight I'll be replacing that
filter, sucking the air out of the syphon again, and I guess adding some
more salt since all that water I added seems to have depressed the SG a bit.