Thank you, You just want to remove the excess thats flat on the glass. You dont want to get into the crevices like the actual glue that holds the corners of the tank together. To do that you would need a wire to slice through it. That really compromises the stability of the tank. This is for the bits you an get to. Take heed that a tank is really good for about 10 years and later it can weaken. Your question is that the top corners are leaking, and wondering if you can leave the bottom? If I were you I would do the whole thing. Look at it this way. If the top has already started to go, do you want to take the chance for the bottom ? It really isnt going to add that much more work or time and I would rather feel its all done then have to do it again.
Reseal A Glass Tank Aquarium