If It is glass & leaking at a seam you will need to lower the water level below that area & use some GE silicone I on it. The problem is If your getting one leak then you may have more, usually glass tanks last many years but It may need re-seaming all of the joints, which would mean emptying the tank, cleaning it then removing the old silicone & replacing it with all new silicone.
If it is acrylic, you need to basically do the same but use Weldon #3 If you can get the thin liquid in the crack, or Weldon #16 to cover the joint from the outside, may be best to get a professional to look into that for you, It may have more issues also.