If i were you id leave the casings out of the water. Even if you sealed them whats to say there wont be some wear to the coating over time. I wouldnt worry so much about the brass rusting as youll be able to see it, but keep in mind that primer has mercury in it. I had a friend whos office window just had a view of the trash dumpsters. He put a frame on the outside of the window and filled it with shell casings. It looked pretty cool from the inside. I think you should put a frame beneath the bottom and behind the back of the tank and fill them up with the brass. Youll still see them through the glass and they wont actually be in the water so its risk free.