You should not have to do anything to a tank to make sure it will not leak.
You should do a 'water test' on a tank when you are setting it up - by filling it entirely with tap water(no rocks, sand or anything else) and making sure there are no leaks. Then drain it, dry it and get to work setting up your reef. This should be done especially if you are buying a used tank. It's also not a bad idea to do it once all of the plumbing & sump is in place.
You cannot add silicone to the edges of a tank as silicone will NOT stick to silicone (and I am sure it would look awful to boot).
Unless you fill a tank entirely - top to bottom & side to side - with rock, there is no reason to be concerned about the weight of the rock or sand you add.
You also want to make sure the tank is completely level - front to back AND side to side and that it sits properly on the stand with no gaps.
There was obviously something worng with the last tank you set up if it was leaking.. the water test should always be done