It is very possible that those shrimp brought something with them. Did you pour the water that the shrimp came in into the tank? Did you quarantine the shrimp before you added them? The fish in the tank the shrimp came from, how did they look?
I know how cold it can get in the Appalachian mountains. We understand that you are doing your best. It could be a water circulation problem. I had a problem where I couldn't keep my tank at the right temp, so I increased the water circulation. Worked like a charm. The whole tank is now 79F.
EDIT: Preset heaters just keep the temp a certain number of degrees above room temp. If there is any way to increase the room temp, that would help the tank water stay warmer.