I had one of my leopards at the vet yesterday, and she was concerned when she saw tiny insects on her. I told her they were in the substrate, and she said to definately cahnge it. I think that they're springtails, but there's another that I don't know. Anyway, she thinks that they cause stress to the tortoises. If I have to change the substrate, I will, but I have about 250 pounds in there, and really would rather not. If I do, however, I don't want to reintroduce. They either came in the coir, cypress mulch, or bags of cheap topsoil (peat and sand). I'm guessing the topsoil.