Sorry to hear that about your snails. I love having them in my tank. The substrate probably should have been rinsed off before being put back into the tank. Have you filled up the tank yet? If so, you can check the ammonia reading. You don't mention having any fish, but unless the ammonia is incredibly high (I honestly have no clue what ammonia concentration would harm aquaria plants), you should be able to just set up the tank like normal, add the plants, and monitor the ammonia and nitrate to make sure they go back down to zero before stocking any fish.