I have an aquarium decoration in the center of my tank that stands very tall. Trouble is the top is sort of hollow (just a crack about 1cm across), and my three clown loaches have figured out a way to wedge themselves up in the top of it. I am worried about their safety and health in there, plus I never get a chance to see my fish because they spend all day hiding and only come out at night (or when I actively shake them out to clean the thing)!
So I'm looking for an aquarium-safe "filler" goo that I can pour into the top of the decoration and leave just the lower cave. Is silicone gel a fairly standard aquarium sealant that would fit my needs?