Originally Posted by emc7
Any black sealant (came before the clear stuff) is likely 20 years old and should be replaced. Any tank than leaks should be entirely resealed as well as any used tank of unknown age.
My brand new 10g has black silicone--it's less than 3 months old...
I think you mentioned something key here, though. If a tank leaks, it MUST be completely re-sealed...you can't just patch it. New silicone will not adhere to old silicone.
You must completely strip the old stuff out (razore blades!) and thoroughly clean it before re-sealing.