Did you acclimate them before adding them to the tank? They are sensitive and require acclimation. I have shrimp and snails in a 5 as well and have no problems. Leaving an algae wafer in over-night is usually no problem. I use only 1/2 of a large tab for my tank. I break up the half into multiple pieces (2-3) and try to keep it scattered in the tank due to the overabundance of occupants. Many times it will last 2 days.
Copper in your system is not healthy and you might want to check for that although the Prime should help with heavy metals. Have you checked for it? Tank needs to be fully cycled and your parameters look good.
Yes i acclimated them. They had already been floating for a while to temp acclimate then i added small amounts of my tank water every 10-15 minutes for an hour or so. I have copper out of my tap, but not in my water change water. I bought some cuprisorb that absorbs copper and i run my water change water through that for a while before i add it to the tank. Even though i have been told by people that they have copper pipes and have no problems. I like to err on the side of caution. So i don't think it is that.