Losing Fish and Scarlet Shrimp

I've had my tank for three years, the water parameters are fine, but I lost four out of six fish in about three weeks, and my Scarlet Cleaner Shrimp. I have two fish left but don't have high hopes for them.

The shrimp stopped eating and "grooming" and roaming the tank. She was three years old, but I knew she wasn't well. She died in two days.

There is nothing in their eyes, mouth on their scales or fins (that I can see). One day they're behaving normally, eating well, and the next they're dead. The only one I could tell was very ill was my Six Line Wrasse, who was swimming well one day and the next was discolored and swimming weakly. The next day it was dead.

I realize no one can diagnose what's happening, but can someone give me a few pointers on what to look for? What fish diseases are there that can't be seen? ANY ideas at all?

The only thing I've done differently just prior to the deaths is I added new sand to the tank - CaribSea Special Grade live sand - the kind I've used since I had the tank. About a week later the first death occurred.

I have a clownfish and a diamond goby now. Today, they seem perfectly normal in their appearance, behavior and eating habits.