Harry is on the mend
Well, Harry is still alive, and he's looking much better! He's still in "hospital" but the black of the fungus on his back and fins is fading, and he's holding his dorsal fin high. He's eating well and swimming around vigorously.
I've been monitoring his progress, and I've been cleaning the water every day (or almost every day), so I think that has helped to fight the fin rot.
One thing I did notice was that the first few days he was in hospital, when I changed the water, he was very loathe to swim around and eat his food. Now however, straight after I've changed his water and sprinkled his food in the water, he's up and at the food. So he is obviously getting better.
For the moment I'm going to continue keeping him in his hospital, until I'm satisfied that he's well enough to go back to the fish tank, and uninfected enough that he's not going to infect the others.