You need to remove the seam that is leaking, clean the glass thoroughly (I used vinegar) and reapply the sealant. Use tape and a tongue depressors to keep the seam neat. Silicone will not adhere to old silicone. Be absolutely sure that the silicone is aquarium safe. GE Type 1 is preferable or some type of sealant that is labeled as aquarium safe. No colors, additives or mildew inhibitors as they will kill your fish.
Why do I love fish? Because every time I stare into the little world I have created for them, every problem I have just floats away....