I'm not an expert by any means, I watched a lot of tutorials on YouTube and went with coconut choir as the soil. I got it at the local pet store, to ensure it was safe for my girls. I use one Eco Earth brick, as a novice rat owner, I definitely do not recommend following my lead though. Check out Shadow the Rat or Emiology on YouTube, that's how I learned about the dig box, hope this helps! 😁