I have a very sad update today. After Alan was doing so well and his holes were healing up, he developed pop-eye. I don't know whether this was another symptom of hexamita, or something else that took advantage while he was ill. All of the other fish are fine.
Having had such success with the octozin I opted to try Waterlife's myxazin to treat him. It didn't help. The night before he went downhill he seemed better than ever, and ate more than I've seen him before! He went back to hiding in his cave on Sunday and wouldn't come out to eat, his colour had turned black. I lifted the coconut once to check on him (I thought that if he did pass away in there that the water flow passing through it may not carry him out and he may get stuck in there). He darted straight under the roof as I lifted it, it seemed as if he couldn't bear the light. This was on Sunday, which was day 3 of a 5 day course of myxazin, so I decided to persevere and finish the course. There seemed to be no change on Monday.
By Tuesday morning he was worse, he had left the cave overnight but was sat in a comer on the sand. He had now turned grey. When I went in with the net he just floated upwards without objection. So I helped him on his way.
Thank you so much for your help. I truly believe that he was feeling much better after the octozin. He was better than he had been for a while.
RIP Alan.