Why did my shrimp die?

I just set up a 5.5 gal to be a breeding tank for shrimp. (I have a thriving colony in my 24 gal, but I'm getting a pair of rams and, since I don't know whether my shrimp will survive them, I want to assure the survival of the species ). Hadn't intentionally put any shrimp in there yet; right now I have about two dozen MTS in there, to start the cycle and because I didn't have anywhere else to put them. When I got up this morning, I saw a bright red shrimp on the moss in the 5.5 gal. She must have come with the moss, which I moved from the main tank. She was very sluggish then, and now she's dead.

I'm trying to figure out why before I put any more shrimp in there.

params:

ammonia 0
nitrates 0
GH 75
KH abt 70
ph 6.8
temp 78

All the same as my other tank except the nitrates (abt 10 in old tank) and temp (81--the lights make it hot in this weather). One of the two small filters I'm running in the shrimp tank has charcoal in it--I don't use that in the main tank. Could that hurt the shrimp?

Any thoughts greatly appreciated--I don't want to move my shrimp to their new digs only to lose them.

Thanks,
Anne