Mainly, rocks that are stackable and that your fish will be able to swim, hide, and navigate through. There are a lot of pics on this site where people have different types of rocks that are all donuts, and just stacked against each other. Myself I went for a cheaper more complicated approach, I got some flagstone from a landscape place, lava rock from a pet store, and I ____ some typical every day white rock from the side of the road somewhere (that I washed and scrubbed thoroughly). I have pics linked in another post "spoiling my fish" if you want to see it. But as long as you can stack and make cubbies then they should be good.