You CANNOT use regular silicone for anything in a fish tank!!!! This will kill your fish almost certainly! there are silicones out there that are made specifically for aquariums. You can get this silicone at any good fish retailer, they may have to order it in for you, but it is nessecery for your tank, it can be fairly pricey (I bought a tube for $12 Canadian) but you cannot use anything else, it has a fairly distinct lemon and vinegar smell to it. I cannot stress how important this is to your tank, any other silicone contains fungicides that will harm all plant and animal life, not only killing you plants and fish, but bacterial cultures.