on my 16 year old miracles tank (domestic brand in canada) I had almost no excess silicone left. over the years it had worn away and been rasped away by my royal pleco. the tank still didn't leak.
the thing to remember is that tanks have two seals:
primary seal (what is actually between the glass only)
secondary seal (all the excess you see in the corners). this does nothing to hold water in the tank. it just serves to protect the primary seal.
Aqueon (aka All Glass) tanks for example, refer to their secondary seal as a 'cone seal' over top of the structural (primary seal) - I just was conversing with their aquarium department over a silicone question I had with my 150g, so I got the scoop LOL. the tanks are 'double sealed' in with two seals as I described above.
So, as long as your primary seal is fine, the tank should not leak.
chances are, as this is an older tank, it was similar to mine -it used to have more of a secondary seal but it got worn away from aggressive algae scrubbing, fish picking, or algea growing underneath the outer edge and pushing up the secondary seal a bit. this makes it easier for it to peel off as the tank gets on in years.
since there is no secondary seal left, its easy to reseal the tank because the secondary seal is already gone.
all you have to do is add your own. you could easily do a reseal on this tank by adding new silicone into the corners if you are concerned. when you reseal a tank, this is actually what you are doing: you strip away all the secondary seal and redo that. you generally never touch the primary seal. it remains intact and you dont seperate the glass panels ever.
hope this helps!
