I treated the tank with methylene blue for three days and haven’t seen the poorly guy at all for days. Did water change today and he’s nowhere to be seen. Have taken all ornaments and plants out and had a look through the sand and can’t find him anywhere. Expect him to be dead but will the other fish have eaten his remains?
Fish will definitely eat other dead fish - nature abhors waste, and why should they turn down a free meal when it could be the difference between life and death in the wild? So while it might seem harsh to human eyes, it's not so strange when you see it from their perspective.
On the other hand, fish tend to be brilliant at hiding as well... I've been convinced I must have lost a fish on a few occasions, hunted everywhere, only for them to appear later, so make sure you look thoroughly a few times before writing him off!