Shrimp keep dying??

I hope someone out there can give me an idea what's going on in my tank. I
have a 29 gallon reef/fish tank with approx.15-20 lbs live rock. It's been
set up for about 10 months. After I had it set up I bought a pepperment
shrimp who got big and was thriving. I found him dead recently in my tank
with no apparent injury. I rubbed it off that he was old. I since had
purchased 2 cleaner shrimps at different times and lost both of them within
a week after buying each of them. Again no apparent injury. Salinity is 24,
PH 8.0, no nitrites or ammonia. Nitrates around 60.(It's always been around
this point since setting it up) I added a few corals to the tank a month or
so ago. I have a damsel, 2 clown fish, clown goby, 2 crabs and a turbo
snail that have lived in the tank a long time and doing well. The guy at
the pet store thought maybe it was a mantis shrimp killing them. I haven't
seen any in my tank and assume they would eat the shrimp if they did kill
them?? Anybody have any ideas what is killing off the shrimps?

Thanks--Susan