6 shrimps in a 72 was not crowded becasue the peppermints hid all the time. The reason I had so many was because I had an apstasia problem and the LFS
was doing a 5 for $20 sale on pepps. I do bi-weekly 20% water changes and do not dose anything for this reason. However, I should probably by an iodine test kit and check it to make sure. I hadn't thought about the shrimp competing for the iodine in the water. Now that you mention it, I did not see many molts from them. That may have been the issue!!
I do not have any predators in the tank unless there is a hitchiker that I am unaware of. I try and keep a close eye on the tank too, so I don't think that is the issue, but who knows.
Either way, when I do decide to add more shrimp I will not have 6 like I did previously. I have no need for any peppermints at this point so I will probably stick to 2 cleaners or 1 cleaner and 1 fire shrimp.
Thank you all.