Could you please answer the following questions?
What are the water parameters currently? (Ammonia, nitrite, nitrate, pH)
What test kit do you use? Is it test strips or liquid drops?
What do you feed your fish?
How does the poop look?
Any other unusual symptoms you have observed?
How long had he been under aggression with other channas?
Was he injured? If so, please tell us the injuries in details.
Have you treated him yet with anything?
Do you have full tank shots?
Assuming your fish has internal parasites, the poop should appear whitish and stringy associated by listlessness, lack of appetite and wasting away. It appears to me this is the exact problem although all the same, we still need answers to the questions above.
If my assumption on this one is correct, then what we need is to treat the fish with metronidazole or praziquantel. I prefer praziquantel honestly since praziquantel is much more effective although it does not come cheap either. I don't know where you can get this in Malaysia but veterinary clinics usually are great sources of such treatment. The treatment must be done at 5 mg per liter and in at least 2-3 weeks. Praziquantel also destroys flukes (a common issue with wild-caught fish and those from open ponds) aside from internal parasites so that comes as a bonus to prevent further issues that may come along the way.
The other powerful dewormers are fenbendazole, flubendazole and levamisole hydrochloride. Just as a precaution, I woud check their sources if I were you so you will not have to hassle about them if you need them.
