I spent the entire evening acclimating him, and turning up the temperature. I hope it wasn't too quick.
I went shopping in a mad rush today, fetched a 3gal tank for Feyd (the red guy who seems to be doing just fine) a heater for each of them (I like to have a spare to fix any differences in water in my water change buckets if I don't get it spot-on) and some cute tunnels, along with a couple of plants for Feyd.
Weehawk (what I've named the little one who's having problems) is still sitting at the bottom of the tank, but I'm sure I've done all I can for him for the moment, having fetched all the supplies, except the food you mentioned (the local pet store here is atrocious, I need to make a trip to my favourite fish store soon).
Thank you all very much for your help, I hope with your advice I can nurse poor Weehawk back to health.