For whatever reason, the pods will die back within roughly 2 weeks. According to Pete Giwojna (noted seahorse expert and author) in a recent response to an Ocean Rider post:
"Sometimes a burgeoning colony of amphipods will multiply to the point that it exhausts the supply of some vital nutrient, which will cause the population to crash."
I would remove the word Sometimes
. For some reason, pods don't seem to stay in abundance for a long time, even without predation, but almost always show up in mass with a new tank. In Sistrurus' tank they are kind of interesting to since they are bright red and large (they love Cyclop-eeze too
No, you do not want a fish in with Barneby. The pods should take care of themselves (Barneby may be eating them and ignoring the crabs). Even just changing the environment of my little guys, it took them a week or two to adjust and Barneby may just be acclimating to his new home.
Your salinity is fine. Be sure you are topping off with fresh water (daily or every other day depending on your evaporation level) and that you are premixing your saltwater for water changes so that you are getting a fully dissolved reading. The only other thing I can think of that might irritate the skin (probably not the case) is your PH. You should be around 8.4 and I think you mentioned that it was initially quite a bit lower. You may want to check in the morning and again in the evening to see if it is changing significantly over the day and SLOWLY add a buffering agent to your top off water if it is not stable. Even using an RO/DI unit, local water buffers differently. Thales is successful with just mixing salt (I believe he mentioned using heavy carbon and DI filtering but not RO), my water will not even come close to 8.4 without buffering.
Plenty more heart attacks to come - keep us posted