Discus Has Suddenly Stopped Eating

[mark4785]

In addition to not eating the Discus is twitching and shaking its head, making a yawning motion with its mouth and the pectoral finds are clamped shut more often.
 
The water parameters show 0 ppm of ammonia and nitrite, a pH of 6.0 and the temperature is 31.5 degrees C.
 
I'm not too sure what is wrong with it. I did recently add in some new aquarium plants which were introduced alongside an anti-helminth medication. The fish isn't flicking, scraping and gill rate is normal so I don't suspect flukes.
 
Any advice? What steps would you take in this scenario?

 

[mark4785]

The fish has now decided to eat so I guess it is just playing games?

 

[Koridora13]

Are his fins still clamped? This may indicate he is stressed out!

 

[WILDER]

Does the fish look healthy and plump?
Do any of your fish look thin, or have sunken in bellies?
What does it look like when your fish go to the toilet. What colour is there waste,
does it look stringy?
Have you ever wormed your fish with a Discus Wormer?

 

[mark4785]

Does the fish look healthy and plump?
Do any of your fish look thin, or have sunken in bellies?
What does it look like when your fish go to the toilet. What colour is there waste,
does it look stringy?
Have you ever wormed your fish with a Discus Wormer?

 
The above symptoms are now back. It will not eat much.
 
1. It looks healthy but could be more plump.
2. Its belly may be slightly sunken.
3. The fish waste is dark brown and isn't stringy.
4. Yes, I wormed the tank with Kusuri Wormer Plus about 2 weeks ago.
 
The lack of eating, shimmies, wobbling of the head and yawning have gotten worse after my 50% water change. The pH of the tap water is around 7.4 but the tank pH is around 6.0-6.5 so it could be that the water change vastly changed the pH. If this 'change' is the cause of the stress, how long will it take for the fish to start behaving normally again since this change isn't perpetual; it happened on Monday?