I got a coconut shell from petsmart that contained a snail in it, obviously snail was removed but could it still contain the black ich? Someone else on this thread told me it may be because they are eating too much spirulina
No - they need to have that third host as well, usually a bird. Also, even if the fish comes in from the wild with this, the disease dies out due to lack of a host.
I saw you might consider using salt. Just and FYI - use marine aquarium salt, not just sodium chloride. The calcium in the sea salt makes it less toxic to the fish.
Jay