Decided in plexiglass for the mend. I have had bad luck with silicone and plexiglass when used on seams. When siliconed like this, however, I have found that it will never cone off. In fact, the hardest part about taking the 75 gal apart was removing a plexiglass filter box I installed over a year ago.
Even if the plexiglass lossens, the silicone will still be there.
I will not rule out a future glass replacement.
Will have to do for now.