is this a new tank? or is it the one you were going to reseal? Empty the tank and make sure it is completely dry. start removing the silicone at each edge do this one edge at a time make sure it is completely clean of any old silicone and make sure the edges are dry. apply a string of silicone down the edge and smooth the edge with your figure, spreading the silicone to goth sides of the edge. do the same to the opposite side edge. leave it cure for 24 hours and then do the same thing to the other two corners. leave them dry 24 hours and then take all the old silicone off the bottom and clean and dry it, then reseal it the same as you have done on each corner. leave the entire tank cure (dry) for at least a week before you try refilling it again. refill the tank very slowly. It should all hold, and you will have a tank that will last for a while.
I can't really tell but it looks like the tank had been repaired once before and it doesn't look like a real good job. It almost looks like someone tried to piece it together. It might be good for a reptile tank but being used as a fish tank. I would have some serious reservations.
Are you sure that the tank is rated for use as an aquarium? There is a difference between reptile and fish tanks, and you don't want to use a reptile tank as an aquarium for fish. Some tanks marketed for use as both an aquarium and terrarium, it's the ones that are only marketed as terrariums that you have to be careful of.