I woke up this morning and Ruby was dead in his tank

He wasn't looking too good yesterday and I was going to change his water again today, but he passed before I could. I got him at Walmart a couple months ago, so at least he was out of that little cup for a couple months.
Fantasia has fin rot also and I'm pretty sure Petco has it but I don't know what it's called. I read a little bit on it and it says that you should isolate the fish that has fin rot because it can be contagious if the other fish is really stressed etc. I've been thinking about getting Fantasia his own tank b/c he's in with Flame and Fantasia's really big so I think I'm going to get another tank for him and treat him in that so Flame doesn't get it. This is turning out to be a great day...