Is the tank glass? If so the brace should also be glass unless it has a plastic rim that the brace attached to. If the brace attaches directly to glass use glass and a good structural silicone (I use Momentive RTV or GE SCS1200) If you are going to use plastic or acrylic attaching to plastic, do not use silicone, it will not give you a good bond. Use Weld-On #4 solvent.