Any potting soil with no additives, ferts, will do but you should take out all the bigger hunks of stuff. I topped with sand. However, after trying to siphon the first time, I added some gravel on top of the sand. What a mess I stirred up. Of course, it didn't help that I had the wrong size siphon. Got a mini, now. In a NPT, unless you have lots of junk, I wouldn't even bother. The plants use the poo and your cories or snails will clean up the excess food, (although my betta leaves no food to drop).