poorly harlequin and other travails

[Catfish586]

I've done a liquid test for ammonia and it's yellow so zero, which is good news. There is a water change due today. Here are two more pics, of the tank overall and one of the restore, who does look a bit fed up without any friends. Ideally I'd give him to someone who has an existing shoal but don't want to do that if he's poorly.

 

[hansgruber7]

Sounds like to me like you may have introduced a parasite of some kind. Others may disagree with this, but in the past I've added the Aquarium Co Op Quarantine Trio when in doubt in these situations and it's solved my problem both times I did it.