Your fish is suffering from what is known as Hexamitiasis. This is not a bacterial infection. As such normal broad spectrum antibiotics won't be effective; anticeptic solutions, much less effective. It is in fact an infection caused by hexamita, a type of flagellated protozoa. It's the most serious illness a fish could get. And it could be lethal. For a time, hexamita-infected fish in Malaysia were put to sleep as a standard measure to end the fish's misery and prevent further infestation. Of course, nowadays we know that there is a cure for this.
There are 2 ways hexamita could attack a fish, either through the gastro-intestinal tract or subcutaneously, that is, under the skin with visible holes to be seen around the head (this is known as the dreaded hole-in-the-head disease.)
The drug of choice to get rid of these protozoans is metronidazole, usually sold as Flagyl, 500mg. This drug could easily be bought OTC at any drugstore and is quite inexpensive. No need to look for hard-to-find magical petshop medications. It doesn't get easier than this.
The actual treatment though is a different matter. I'll give you a step-by-step process.
1. If you're using an overhead filter, remove the carbon if any. It would be ok to leave the glass wool in the filter box.
2. Perform a 50% water change. Syphon your tank until it's half empty. Replace with 25% chlorinated water and 25% conditioned water. (My definition of conditioned water would be that which has been aerated for at least 48 hours and dosed with aqutan.)
3. Dissolve the metronidazole tablet/s in a cup of warm water. Don't skip this proceedure. The water must really be warm. Metronidazole works best when dissolved at a temperature of 90+ degrees. A cup with 50%-60% "cofee-hot" water plus 40%-50% tap water would be fine.
4. Let the tablet/s dissolve for 5-10 minutes. Afterwards, mix well.
5. Drop the metronidazole solution in your tank. The required dosage would be 500 mg. for every 10 gallons.
6. Repeat steps 2-5 after 48 hours. You'd need around 7-8 doses of metronidazole. Usual treatment would be from 10-15 days. Observe the waste of your fish on a regular basis. If your fish shows improvement before 10-15 days, there's nothing wrong in finishing the entire cycle. Extend treatment if neccessary.
If you have a heater, set it at 30-32 degrees. Hexamita thrives in cold water. While the medication used to kill them works best in warm water. For those with no heaters, don't worry. Its not absolutely neccessary to use one. Just be sure to dissolve the metronidazole in warm water.
If you've got more time to treat your fish then you could apply the following protocol:
1. Again do steps 1 through 5.
2. After 24 hours, implement a 20-25% water change.
3. Drop your metronidazole solution but this time at 250 mg. for every 10 gallons. Repeat steps 2 & 3 everyday until the fish recovers.
By the way, the very best food for a fish recovering from a bout of Hexamitiasis would be frozen bloodworms. If the infected fish is still eating, a feeding of pellets pre-soaked in a metronidazole solution would make it recover quickly.
If the fish is suffering from the external type of hexamitiasis, the hole-in-the-head disease, the same procedure above would cure it. In this case, malachite green may be used in conjunction with metronidazole primarily to prevent secondary infections.
This illness is quite common nowadays. Dip your finger in the aquarium in the evening and you'd get an idea of how cold it gets. I hope this post would be helpful to a lot of you out there.
There you go.
. How ever it can also be cause by bad diet so u should feed him different food every day I feed him krill/bw/flakes/cichlid tablet. 
