If super glue will infact hold (I'd use the gel), it's safe for tanks. It essentially dries as a plastic.
We used superglue all the time at work to fix broken impellers and never had any issues... other than the super glue eventually failing and having to reglue the thing
Edit: if that won't work I've got one other alternative for you. A friend and I did a set up where we built a wall of driftwood using many small pieces. To secure them together we drilled holes through the wood and put acrylic rods/strips through the holes. You couldn't see where we had drilled/braced and the wall held together perfectly.