Royal Gramma died tonight

[rocketlily]

I have had a Royal Gramma for just about 3 weeks now. He adjusted to the tank (20 gal) very well and within 2 days was coming out, feeding well and exploring the tank. Monday night he ate well and was fine. Last night he came out but did not eat. Today when I came home he came out and layed on the sand in the front of the tank and 2 hours later, he's dead.

All parameters in the tank are excellent. All the corals and snails & Emerald crab are fine. The Royal Gramma had no spots or any visable sores or spots. He is the only fish in the tank and I have added nothing new since I got him.

I'm baffled. Any ideas?