Pegasus, my apologies if you feel insulted, it was certainly not my intention. I have made many tanks and because of that generally have a good look at the way other people make them and have never yet seen a tank around here with an obvious gap between the glass. It may well be that I have been in the Chathams and have eaten too much crayfish. My intention was to find out if there was a reason why people left a gap and I had assumed that it was for the reason you gave. My problem I guess is that in a previous life I worked as a rubber technician where I learned that adhesives and fillers are not necessarily the same thing and I was curious as to wether the advantage of the added flexibility was outwayed by any weakness through having a greater thickness of RTV. You would know that RTV sets in contact with the air and I believe that too much RTV will not cure properly in the middle.