Hello. I originally posted this in the betta section but got no replies, so hopefully I will get more response here. My betta has developed an ulcer or a sore on his side. He is in a 48 litre tank with six khuli loaches and I have had him for about 15 months. Ammonia 0, nitrite 0, nitrate about 15. This is what it looked like on the 17th Jan:

I treated the water with Myxazin on the 17th Jan as it says it treats ulcers and sores. I treated him for five days and the last dose was a week ago. It didn't seem to help, and actually seemed to get bigger:

It's been a week since the picture above was taken and the sore is slightly bigger now. He doesn't swim about at all, he just spends all his time hanging about at the top of the tank not really moving. I also think he is beginning to look a bit bloated. He is still eating, but I have not been overfeeding him. Does anyone know what this is and what I could do?
Many thanks for your help.

I treated the water with Myxazin on the 17th Jan as it says it treats ulcers and sores. I treated him for five days and the last dose was a week ago. It didn't seem to help, and actually seemed to get bigger:

It's been a week since the picture above was taken and the sore is slightly bigger now. He doesn't swim about at all, he just spends all his time hanging about at the top of the tank not really moving. I also think he is beginning to look a bit bloated. He is still eating, but I have not been overfeeding him. Does anyone know what this is and what I could do?
Many thanks for your help.