check the ingredients list. There are a lot of different types of "dirt" that will work. The old suggestion of the miracle grow organic choice dirt was great; but they seem to have discontinued that exact product. What was good about that old miracle grow stuff was what was listed in the ingredients. Yes you want organic; but thats quite vague. The important stuff in there was "poultry litter" (chicken poop) as opposed to cow manure found in many organic dirts. You could even use a brand of dirt that has cow manure; but you'll likely get ammonia spikes at the beginning that you'll have to wait out before adding fish. You can also mineralize dirt right from your own backyard (and/or purchase mineralized topsoil). THe process basically involves spreading out a bunch of dirt real thin on something like a tarp. Then you get it wet, mix it up, and let it dry out. Now repeat many many times and you've got mineralized top soil that should work perfect for a dirted tank.