It has been a few years since I used it, and there may be newer varieties, but there is a pour in coating system called Kreem (I think). I think it was developed for steel tanks, but should work. The kit has multiple bottles in it, one of which is a cleaner/etcher that probably shouldn't go into a 'glass tank. Hint: Read the instructions carefully! ALSO, remove the petcocks, and plug the fittings. Moving on, grab a clean bath towel or small blanket to lay the tank on. What you need is to be able to set the tank so each inner surface is level at some point. Bunching up the cloth or adding more rags, etc. The process is to pour in the heavy liquid, slowly rotate/tilt the tank to coat all surfaces and then lay it down for the specified time. You then repeat, but lay it down with the sealer sitting on a different surface. This continues until you run out of liquid sealer, or liquid refreshment.
You did remember to coat the area that leaks, yes? The problem with 'glass tanks is the defect on the inside may not be directly opposite the external leak. So, if you decide to do a repair, the patch should be larger than the visible damage. BTW, with a 'glass tank, if the leak is a crack near the mounts or petcocks, it should be patched first. Otherwise, the crack will continue to travel. You can then seal it, for a belt and suspenders approach :wink: