Hi and welcome to the forum
What colour is the fish meant to be?
It appears pink in one image and blue green in the other and I have not seen Betta these colours before.
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The easiest way to remove ammonia from an aquarium is with big daily water changes. Basically you replace 75% of the water on any day there is an ammonia reading above 0.
Make sure any new water is free of chlorine/ chloramine before you add it to the tank.
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Pimafix and Melafix should not be used in tanks without aeration/ surface turbulence, or in tanks with Labyrinth fishes (Bettas & gouramis) because it can leave an oily film over the surface and can harm the fish.
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The patch on the side of the body looks a bit like an ulcer, although it is the wrong colour. Part of the dorsal (top) fin also appears to be damaged/ missing in the picture of the green coloured fish. This is probably a bacterial infection, however you should try to get rid of the ammonia problem first because that can weaken the fish and make them more susceptible to health issue.
Quite often the easiest way to treat sick fish is with daily water changes and this will also help with the ammonia problem.
I would stop adding medications and simply do a 75-80% water change each day for a week and see how the fish looks after that. If it has improved a bit then keep doing daily water changes. If it gets worse during the next week then post another picture and we can talk about different medications.
You can use any sort of plastic or glass container to keep fish and generally the bigger the container, the more water it holds, and the better it is for the fish. This is due to nutrients being diluted out more in larger volumes of water and more concentrated in smaller volumes of water. A larger aquarium/ container would have lower ammonia levels and be easier to treat if it needs to be treated.
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The following link has some information about what to do if your fish gets sick. It is pretty long and boring but worth a read when you have some spare time. I recommend printing it out and reading it in bed to help fall asleep.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/