As Lynda asked: what did you use for an ammonia source for those three weeks? If you added the bacteria starter and let it run without an ammonia source it isn't cycled. Do you have a test kit to test the water now? If it's fin rot, it's usually caused by poor water quality. What is your water change schedule (how often and how much) and what dechlorinator are you using?
I would start with a 50% water change with a good dechlorinator. FIn rot can sometimes clear itself up with very clean water. That white thing on the fin though, not sure if that's a fungus or if it's getting infected. Again I'd start with a water change and keep your water clean and test the water daily. If you can test the water and let us know the exact numbers for ammonia, nitrite, nitrate it'll help us pinpoint the problem.