Originally Posted by adrenaline
I tried those twice and each time it came to an unhappy end. They died.
Black skirt tetras are usually a pretty sturdy fish. If they died, I would be curious about how the situation played out, thus maybe helping us to understand why they died, and helping you to try again with more success. If they were sick when you first got them, that would say a lot. Please make sure that when you select fish that they are healthy, and that the other fish in the store tank are also healthy. If you see dead or sick fish in a store tank, stay away from anything in that tank.
As for silver dollars, you could begin raising 1 or 2 in a 55 gallon, but be prepared to upgrade to a larger tank as they grow. Full grown, silver dollars will reach a size of about 10 inches in diameter... and they can be quite messy where waste levels are concerned, so lots of small water changes is also a good idea. Aslo, be forewarned, silver dollars are heavy vegetarians, so I would not suggest adding them to a tank that has or may have live plants in it.
As Lupin suggested, if you give us more information about what traits you're looking for, maybe we can help you to make some informed choices that will work for you... What other fish are in the tank if any? The more information you can provide for us, the easier and faster we can help.