Here's my input, since I started fishkeeping awhile ago every single fish I have purchased from the LFS - except for some neons and 2 male pearl gourami, have all died within a really short period of time after purchase. All of the fish I keep now have all been bred in NZ locally and are all really healthy and active. All my water perimeters are fine and within the suggested range for the fish I keep and yes all of my heaters work and I have separate gear for all 4 tanks.
I haven't noticed any scales missing but I do have the other symptoms that you guys seem to be having, there has to be some common denominator here surely! I am beginning to wonder if all the fish we are losing have come from the same supplier/importer? I am pretty sure that Wonderworld in Rotorua is our LFS supplier but don't quote me on that, they wouldn't tell me when I just rang them as they don't want me cutting out the middle man so to speak, even after I told them why I wanted it.