I've just got new velvet slugs but they don't seem to be moving much I've heard that you should keep them like African land snails which I have two of but the slugs don't seem to be touching the food I've put in. I've had them about a week now (got them online but the guy was rude so don't want to ask him) and they are very small. They are kept in a plastic tank. Am I doing something wrong?