If it's between the panes of the glass, then a more complete / safer fix would be to separate the panes and redo that section of the tank. Most of the structural integrity of the tank, according to someone I spoke with from Aqueon is in that bead of silicone between the panes (granted, the frame helps to some degree and the interior bead helps to a lesser extent).
If it's a small tank, then you might be able to get away with just resealing the interior bead. If it's a larger tank, I would break the seam and completely fix it.
For a small tank, you can use the DAP aquarium sealant. It's sold at Home Depot around here.
For a larger tank/bigger job, you'll need a tube of silicone. Do not get the kind that has any colorant or anti-mildew added. You may even be able to find a tube of the Marineland aquarium silicone at a Petsmart if you're lucky.