So I bought a cube that I've posted a few threads about. I've got it plumbed and it's water testing at the moment, but the most baffling part of the tank was the back panel of glass. The cube is 1/2in thick glass all the way around, but then there was a second pane, maybe half the thickness, silicon'd on top of the back pane. This struck me as odd, but I shrugged it off. Then water started leaking between the two panes from the bulkheads. So I replaced the bulkheads, but decided I was definitely not going to deal with this in the future and cut the thinner pane out. The bulkheads fit far more flush now and I have no leaks. Turns out the holes that were drilled were not perfectly aligned and this was causing some of the seal issues for the bulkheads and allowing water to get between the back panes. That and probably just old bulkhead seals. The question I keep asking is why would they do this? The second pane is clearly DIY after the fact. The second pane was slightly tinted, maybe it was for that effect? But the original panel was also painted on the back, so it's not like it mattered in terms of background. The original seals are all fine and this would seem like the worst way to fix that if the original seals had been compromised. Anyone got any ideas why this would have been done?