I have an auto-topoff setup - which has been a huge time saver. But I found out the down-side to it this week.
My protein skimmer (CSS125) went nuts, and was slowly pumping water into the bucket it drains into - all night long.
That meant my auto-top-off was replacing all that with fresh water.
By the time I noticed, the next morning, my SG
was down to 1.016. I bought some new salt, and by that night I moved it back to 1.018. I waited a day, then I pushed it back up to 1.020. Today (after another day's break) I'm working on pushing it up to 1.022 or 1.023.
I've been dripping a high concentration of saltwater into the tank, off and on, for the last 3 days.
I'm pretty sure my Xenia aren't going to make it - they look bad. Grey and shrivelled.
I have a brittle star that rarely comes out during the day, sitting in the front of the tank, and he's barely moving.
Everything else seems fine - the 2 anemones, fish, zoos, gsps, etc.
I can clearly see the advantage of having an in-sump skimmer now - plus, I need a way to handle overflow situations if and when it goes nuts.
My new 180 (which is going into the cabinet next week) will have an in-sump skimmer, but I'm still going to be maintaining this for a few more months, and don't want this to happen again.