Advice Using An Underwater Sealant

Hi all,

I’m the proud owner of an Aquaoak 220l tropical tank and have a really nice silicone rock background. The trouble is the top left corner has come away from the glass and I’m concerned this will ‘spread’ if I don’t do anything. Suffice to say I really don’t want to move all the fish and completely drain the tank. That would be a nightmare! Does anyone have suggestions as to an adhesive I could use that will stick the silicone rock background to glass which will cure underwater? The only idea I have is to partially drain the tank (ie just below where it has come away), however, it will still be in a damp environment.