Clamped fins are not a good sign. I lost two fish from this (both platies, both female) in the last month.
They started off a lot like that, refused all food and clamped their fins, and due to circumstances beyond my control (a holiday and being flooded in) I couldn't get treatment until it was too late. So I don't know what cures it and what doesn't. Mine became gaunt, hardly moved and dropped stringy white poo - a sign of internal infection or parasites. I knocked the last one out with clove oil and went over it carefully - nothing wrong on the outside other than being very skinny. She recovered from the anaesthetic but died two days later. The other one died about a month ago. She had scales lifting off when she went.
Mine were much, much skinner and more lethargic when I lost them. If it's the same illness you can probably save her. My best bet would be internal bacteria med. I've never used one, so try the Interpet 9. A lot of people say that works.
What is the fish that looks like a black golfball that went chasing after the platy in the video?