Acrylic, Seams

[Whatigot]

Hey Gents, I just bought an acrylic sump that turned out to have been dropped on one of the corners and is leaking very slowly from a 1/2 inch crack just above the bottom seam and about 1 inch from the corner, stupidly I tried regular aquarium silicone to stop it but it didn't work out.
Where can I snag some of this weldon stuff?
I had heard I needed to go to a specialty retailer to get something suitable for my problem?
thanks a bunch guys.....