Hi, weight wise its hard, but if you have the help to turn it and the help to silicone it, i would stick right under the other. BUT, definitely silicone ontop and underneath the crack too, do a line over the crack, lick you finger and push the silicone into the crack. If by smashed you mean millions of pieces you should remove it due to the likelihood of fragments falling into the water. If theres one of those fan shaped cracks with 10-20 or so lines coming from it you could silicone the piece underneath but also silicone a pice inside over the cracked area too, draw a square and lines oner the main cracks and squish down real hard. Will never be fool proof but if you use the overkill method it should work. I have siliconed a crack in the base of a 2ft deep hex before now, 1 crack, i cleaned it, dried it, and smeared a thick line above and below the crack and then tested it.
This isnt guaranteed to work, only worth it as a 7ft tank isnt cheap! a photo of the tank would be good so we can see the damage, also your story as we all learn from others mistakes and accidents etc.
Rick
