Shells are what they like best of all. Old apple snail shells work well, and so do edible snail shells (you can buy these at gourmet food stores, along with a can of snail meat). Seashells are good too, but they tend to look a bit weird in a fresh/brackish water tank if too bizarre. So go with the rounder shells like naticids rather than obviously marine things like conches.Space each shell about 5-10 cm apart, and your BBGs will use these quite happily, as hiding places as well as spawning sites. In fact, I think the cover of my book has a pair of BBGs laying eggs in an apple snail shell.Cheers, Neale