I agree with Ralph, piano music is the best for relaxing.
If you enjoy piano music you have got to listen to Charlie Sneller. His soothing piano music is perfect for the aquarium. I own 5 of his CDs and my favorites are "Summer Breeze", "Colors of Fall", and Songs for a Quiet Heart". I was told he makes background music for the movie industry.
Charlie Sneller Website:
http://www.inspirationalmusic.com/index1.html
Disclaimer: I have no financial ties with Charlie Sneller, I just enjoy listening to his music.
Addendum: Morgan or Michael, If it's inappropriate to post the link please delete it.