setting up a lake tanganyika biotope

[zeniko]

You can use pretty much any rock you like, I personally used lava rock and it's worked out great.

[mizonix]

For a proper biotope, limestone is the way to go, but I've had no issues with slate or granite. Just make sure to create lots of caves in different sizes for territories and hiding spots. Slate's actually pretty great for this, as it can be split into thin layers with a hammer and chisel, allowing you to create plenty of caves without taking up too much water volume with rock. Really, anything will work, even a Malawi setup with a few tweaks like adding a shell bed if you want to keep shellies.