Black silicone sealant is used for a variety of applications in the aquatics hobby. From sealing a cracked aquarium to binding decorative rocks together to form a waterfall, silicone sealant can do the job safely and permanently.
It also has the benefit being easier to undo if you want to make changes. It can be scraped and cut off and in most instances you have 12-24 hours before the silicone cures. It is completely safe, once cured, for all aquatic life. It can also be painted.