Yes, this I knew, heavier than Oxygen at least. I carefully removed the aphids from the upper reaches of the plants, sort of to see if they would climb from below but none moved. I have since cleared them all off, what I could find anyway.... I suspect that I ran into Boyles Law when after several hours, I noticed that the sublimation of the dry ice had ceased. There was still a good sized chunk left, but there were no observable vapours coming off of it any more, (I know they are invisible, but mixing with H20 vapour makes them ever so slightly visible) and there was no sound effects either. There had been a faint hissing/sizzling/cracking sort of noise. The instant I hit it with hot water though it all began over again and soon the remainder was gone.
BTW. most of the problems I have been having stem from these aphids hiding in the moss and dining on the plants at that level. it was only because of some of the more adventurous individuals, and the obviously aphid damaged growth that I noticed them at all, and by then it was pretty late.