Any strong construction adhesive should do, but they do sell specialized tile glue at home depot.
I'm not exactly sure what you're planning, but a structure out of tiles will be super heavy. Its something you'll need to build in the tank for sure. Also, I wouldn't expect to build anything more then a simple basking platform...which may be all you want to build. I have seen some pictures of basking platforms made out of stacked tiles...might want to give the viv pics only thread a gander.
Good luck!