I never dose my plants with fertilizer but I have heard of people experiencing problems with their shrimp that they could only blame on the ferts they where dosing with. Almost all ferts have small doses of copper contianed in them, so I would check all the componants of the fert your using and just see if any copper seems a bit high. You didn't say weather or not you are dosing with Co2 as well as ferts, Co2 can really muck with the pH and kill shrimp.
It really does sound as though posion of some sort has got into your tank, either via new plants, or even something as simple as a chemical getting sprayed near the tank and drifing into your tanks system.
A friend of mine kept complaining about his fish dying all the time and it took me a while to click onto what was causing the deaths. He had one of those automatic airfreshener things that squirted out scent at predermaned times. And the air freshener thing was positioned on top of a low wall pointed directly at the tank. The fish didn't die immediately but it took a gradual toll on them. I am not saying that you have a similar air freshener I was just giving an example of how the most innocent of things can be a fish keepers down fall.
Also just out of curiosity what temp is the tank running at? Too hot or too cold can kill shrimp just as easily.