Well,.... Lets see... I've kept shrimp in a 1 gallon without too much problem. They *are* tropical, and should be kept at around 74-76F. Any lower, and you're going to have higher fatality rates.
I've had recent problems w/ ghost shrimp in my 20 'long' dying a day or two after tossing them in. This is the only tank that has this problem. They live just fine in all of the other tanks. I have also tried dosing Iodine (iodide, whichever is the crud they sell for reefs), which does not seem to help.
My current theory is as follows (based on my similar problem that I am tackling):
1) Low oxygen (Since the shrimp cannot swim to the water surface and 'idle' to breath easier, a lack of oxygen could kill the shrimp, but leave the fish alive).
2) Co2 fluctuation, pH (If I disconnect my CO2 from my powerhead and instead use a airline to bubble air through the powerhead, I have significantly lower shrimp death, although crappier plant growth).
3) Trace element depletion (Perhaps there is something outside of iodine needed for these buggers and the plants are absorbing it too quickly, or a plant is metabolizing it into a form that's toxic for the shrimp)
After personally witnessing multiple times the shrimp freaking out, swimming up towards the surface of the water, and dropping dead, I'm guessing #1 or #2.... I did have a crawfish that parked his gills right on top of the Co2 airstone. He uhhh, kinda stopped moving and seemingly passed out on top of it. Poking him a few times to dislodge him and move him away made him quite a bit happier, and he started roaming around again afterwards.
Or... *shudders*, maybe its a shrimp parasite?