Hey Patty, I'm sorry for the loss of your little one.
What type of water are they in? Is it tap? Treated or untreated for chlorine/chloramines? How often is it changed? What are they being fed? What's the water temperature? Did the one who passed away appear 'bloated' or show any appearance of skin discoloration? There are a lot of variables at play here, and to get a better idea of why/how the first one died and how to keep the other one healthy we'll need to get a little more background info from you.