I do know, both from personal experience and from a story related by a friend that skim-mate can be lethal to a tank. I got lazy a couple months ago and didn't clean out the cup on the skimmer in my 180G tank. Suddenly a tank that had been running flawlessly for months, fish started dying. Nothing new was added to this tank at all. The normal feeding and fish that had been happy and healthy for months. Suddenly BOOM. dead, dead, dead, dead... Finally realized that I had allowed the skimmer cup to overflow with skim-mate. Cleaned out the cup immediately and voila, problem solved. Not a single loss since that date.
I am happy that I know how toxic this stuff is, but very sad that I lost several of my favorite fish to learn this lesson.
A friend of mine, who runs a coral farm a couple hours north of me, related a story where he accidently knocked over his full skimmer in the sump. Dumped all the skim-mate back into the system and virtually wiped out 3 of his frag tanks in one fell swoop.
So I wonder, knowing full well how toxic this gunk is, should we be focusing on keeping the main jar as clean as possible? Or, at the very least, add cleaning to our more "less often" maintenance like rodi filter replacement day...