Hmm, sounds like everything checks out. How many times have you tried to add more snails and crabs? Are they from the same source? Sometimes you just get bad batches. I have had a whole bag of astraea's die while my margaritas and nassarius were fine, all acclimated at the same time in the same way. I just think the astraeas were bad, maybe sick or something. But since you say they die within 5 minutes of being in the tank, it sounds like it must be related to the water somehow. I would take your water to an LFS and have it tested. Have them test everything, in case your test kits are off, and then check things like copper. I also think iodine in high concentrations is bad for inverts. Also, you havent mentioned your alkalinity. Make sure that checks out. Other than that im stumped...good luck