If they look like they're "panting" you need to check out your water parameter situation and temperature ASAP. Take a sample of your tank water to your local fish store, have them test it, then WRITE down the results and post them back here. Don't just let them tell you the water is "fine".
"Panting" fish can mean there isn't enough oxygen in the water or your tank is having a mini-cycle, both serious issues you should take care of immediately, especially since one has died already. Also, silver dollars get very very large, the size of dinner plates, what size tank are you keeping them in? At least a 55 gallon by themselves I hope.