I guess this is a used tank. I can't imagine buying a new one that needs to be resealed. If it were new I'd ask the dealer to get you one that's sealed properly.
The problem with sealing any tank, new or used, is getting the sealant to adhere to the glass. If it's not prepped properly it'll just peel off.
Like mentioned before, use a straight edged razor blade and go down the corners of the glass removing the sealant from the corners without destroying the sealant between the glass panes. Clean whatever you've left with acetone placed on a cloth or paper towel. Let it dry. Cut the tip of a caulking tube to the size bead you want and caulk each of the corners. Place your hand in a plastic bag and run your finger or thumb down each of the corners leaving the size of sealant you wish. If you want the surface to form a skin quickly, use a hair-dryer and go over each of the corners. Let the tank dry several days before filling it with water. Test it OUTSIDE before using it inside.
You might also consider running a bead where the trim meets the glass inside the tank completely around the inside. That would prevent any wicking action when the tank is full.
