[Bombay]

Don't use silicone in a tank with water.  Silicone is poisonous until it cures (24 hours).

Instead, use an epoxy like you can find at www.thatpetplace.com.  Do a search under puttys and sealers.

AQUA STICK EPOXY PUTTY STONE GREY 4 OZ
Excellent for attaching live rock, stony corals, gorgonians, aquatic plants, and other landscaping items. Non-toxic to fish, invertebrates, aquatic plants, reptiles, and small animals. Works on dry, damp, or wet surfaces, and cures underwater. Adheres to glass and bonds to wood, glass, concrete, ceramics, acrylic, metals, and fiberglass. Aquastik can be drilled, sanded, sawed, and machined. Safe for fresh and saltwater.
You Pay: $ 7.49----In Stock: Yes

HOLDFAST EPOXY STICK 4 OZ AQ SYSTEMS
BONDS ROCK, CORALS, SLATE & MUCH MORE. SAFE FOR FRESH & SALTWATER, NON-TOXIC TO FISH & INVERTEBRATES. MIXES IN 1 MINUTE & CURES UNDER WATER. CAN BE SANDED, FILED OR DRILLED AFTER 1 HOUR. USE TO BOND LIVE ROCK & CORAL. USE TO BUILD WATERFALLS & CAVES.
You Pay: $ 7.99----In Stock: Yes



You don't have to buy it there, but make sure you get something that is non-toxic and works under water.

You could also rig something up with suction cups.

Bombay