I kind of wish Tango didn't change. He was such a gorgeous scarlet colored orange :( But I still love him all the same. He's a great fish!
As for Whisper, his fins just keep getting curlier and curlier. Some people say that's due to over-crowding, but there's no way that's what's causing this issue! I bought him from wal-mart in those little tiny containers (because I felt sorry for him) and his fins were already slightly curly. You can see in the last two pictures what he looked like before, and what he looks like now. He now has 3.3 gallons all to him self and you're going to tell me that his fins are getting curly because he's crowded? I doubt it. I believe it has to be genetics.
Maybe it's hard water, but in my area hard water in unavoidable. The water is just naturally that way. So, the fish that are bred here are also acclimated to it. I wonder if there's another thing that can cause curled fins?
Fin curling can be caused by poor water quality. Being in the small cups at the pet store can cause fin curling, like you said. Once the fins are curled, they usually stay that way. I've heard that if you put their tank in the sunlight, it can help uncurl them. I have a VT who's always had a curled bottom fin and as his fins grew, it sort of uncurled, but he still has the little curl. I don't mind it though, it gives him some character.
As I look at Tango's pictures, his seem to be curling too.
In the late afternoons the sun does come through one of my windows and hits their fish tank. It's been causing an algae issue, so I'm going to be getting some otos soon to fix that.