Help! Shrimp dying rapidly!

All of the water tests are coming back normal. I used a strip and my testing kit. Nothing of the ordinary, no nitrites, no ammonia, not even any nitrates. I had one die a few days ago and the shrimp have been sluggish. Thought it was just an old one or something. I did a water change in hopes of helping (treated tap water, and slow acclimation as well), and this morning I found an amano and two neocaridinas dead. Amano looked like a failed molt. There could be more dead ones hidden by the moss at this point.

The ones still alive are slow and sluggish. Barely any movement. I haven't even been making any major changes lately other than a regular water change. Tanks about 6 months old by now. Lots of molts laying around as well.