Silicone doesnt adhere to acrylic very well. Good enough for baffles in a sump, but I wouldnt trust it as a tank brace. Acrylic glue wouldnt do much since it works by disolving two pieces of acrylic so that when it dries, it actually forms a bond between the two.
If you know the manufacture of the tank, I would suggest checking to see if they have a replacement brace.