I have seen this before. My question is how secure is that shelf. If you put another or the repaired tank back on this shelf, will it leak again down the road?
Just the water in this tank weighs at least 200 lbs. If the shelf sags a little in the middle, the vertical glass panes will slowly separate from the bottom horizontal piece of glass.
I have repaired a 30 gal tank that was placed on a dresser. The top of the dresser eventually sagged and the tank of course leaked. I read about this scenario quite often. Some people place their tanks on styrofoam, so that any small irregularities can be smoothed out. But that can only handle a small amount of unevenness . The pink stuff is the best. The better and stronger the base the better.
And I am assuming your shelf sagged a bit.
The best, of course is to take the leaking pane of glass off of the tank and and resealing that part. Plus I would take off all the inside silicone and resealing all of that as well. You could just try to reseal only the insides, and put water in to see if it will leak, but this is not recommended in my opinion .
As Shao said, clean every piece of silicone off and clean with alcohol or acetone in a well ventilated place, like outside. New silicone does not stick to old silicone properly.
J&L should sell silicone specifically made for aquariums, no added chemicals.
quick google search turned this up
https://www.loctiteproducts.com/en/k...fish-tank.html