It is my understanding that once it's cured, all silicone is safe for aquarium use. The main thing you have to concern yourself with is what kind you need for what application. For example, not all silicone will adhere to glass and not all will adhere to acrylic and in circumstances where you are working with glass and acrylic, such as a glass tank that you are going to DIY into a sump with acrylic baffles, you have to make sure you use a silicone that will adhere to both.
I could be wrong but read a lot on this on RC when we drilled our tank and installed an internal overflow box.