Can you find exactly where the leak is? Glass or acrylic?
This is a long shot, but sometimes with glass tanks, if the water level is up a bit high, and the silicone seal on the top trim isn't perfect, water can seep out (over the glass, under the trim) and you'll see a bit of water running down the glass.
Solution - simply drop the water level a little bit.
If it's an acrylic tank and the leak is up high, you can probally drop the water level and seal the damaged area with a bit of weldon #16. It'll have to cure overnight, and the repair will look a bit messy, but it would most likely work.
If it's a glass tank, the same repair may work, just use silicone sealer instead of weldon. Either way, you'll need to use powerheads or something to maintain circulation in the tank while the water level is dropped overnight.