Thanks for your input, Colin.The fish is either a dud or severely stressed out from the move.
Don't add stuff willy nilly because most of the time that makes it worse.
DON'T ADD ANTI-BIOTICS UNLESS YOU KNOW WHAT THE PROBLEM IS.
Did you check the ammonia level in the bag when you got the fish?
If that was high the fish is suffering from stress and ammonia poisoning and will require time to recover, assuming it does.
The best you can do, is keep the tank and filter clean, and contact the company you got them from. Let them know one fish isn't doing well but the other 2 are. Then see how it goes over the next week or so.
Chances are you have 3 male gouramis and that could be an issue too if the other males are bullying this one..
I must admit it would never have occurred to me to check the water chemistry of the bag they came in.
I had already decided all three were males. The place I ordered from doesn't allow you to specify the sex in the shopping cart but there is an 'instructions' section where I wrote that I wanted one male and two females. This was either ignored or else they couldn't sex them. I don't know if this fish was bullied before it was sold to me, or if it was bullied en route, or if it will be bullied in the future should it survive. However, it has not been bullied since I put it in the tank. In fact, the other two completely ignore it - even swimming past within an inch of it as if it wasn't there.
I'm not sure what good contacting the supplier would do, other than giving me the satisfaction of being pissed at them. The most I could expect would be a refund of the £6.36 the fish cost, which doesn't really help me and certainly doesn't help the fish. I suppose it's possible they might have some useful advice...
I am keeping a close eye on water chemistry, being prudent about feeding, and today is filter cleaning day anyway...
The only change in the gourami's behaviour is that it has managed (either accidentally or by design) to wedge itself into a plant, allowing it to maintain an upright position without effort. I have to look quite closely to check, but I can see its gills moving, and its fins wave occasionally.