Help with Shrimp

I have a new Aquarium. 75G with Wet Dry. and I just finished my cycle. I
used Fiji Live rock to do the cycle. I have introduced 3 tangs, an royal
gramma and all are doing fine. I also introduced 2 Emerald Crabs and they
are also fine (though hidden).

The question is I have tried introducing 8 cleaner shrimp and all have died
with one hour of being in the tank. i would appreciate any knowledge of what
could be causing this. I have not used copper pipes or any shapre or form in
any way associated with this tank. What I have done is used PVC pipe (and
glue) for the water supply and return from the filter.

Salinity is 35% (I use a salinity meter not Hydrometer)
Ammonia is undetectable
Nitrites are undetectable
Nitrates are 2-3 mg/l