It's always a worry of mine with a used tank. I examine it heavily to make sure there are none, check the sealing and stuff...but you never know. That would suck, big time.
From personal experience, SUCK is an understatement.
I re did the silicone 4 times before I gave up and just bought a brand new tank. I had only paid $75.00 and they delivered it to me. So it wasn't a huge loss and the new tank was only $99.00, it didn't break the bank.
But all that water everywhere, that's another story.
I would wait a week before refilling it with water. Then I'd wait another week to see if it would leak, before adding the gravel and decorations.
It still took a day or 2 before it would start to drain out the bottom of the tank, every time.
Turns out the tank was fine with the water, but the added weight of the gravel would cause the tank to bow and pull away from the silicone.
It didn't matter how much silicone I used or how far I spread it out from the edges. It would eventually leak all over the carpet.
Fortunately I own a carpet steamer cleaner. So I could suck up all the water in 15 minutes, before it could do any permanent damage. I learned years ago that you should never own pets, even fish, without owning a steam cleaner.
I rent, and I don't ever want to cause that kind of damage to someone else's property.
But the same goes if I owned my home.