This is a long description, you can just go down to the photos if you don't want to read this.

In the last few days my 15 gallon tank has been in the shits! I lost 3 fish in 12 hours! There was ammonia in the tank and the water was cloudy. Yesterday morning I did a 70% water change and added salt. I had two platys and my betta. One platy was perfectly normal but the other was very skinny with clamped fins and was laying on the bottom. My betta had clamped fins and looked very ill, I was sure they were going to die. This was shortly after the water change. After an hour or two I fed them (I hadn't fed them the day before because I am keeping the nitrates and ammonia down) I knew it probably wasn't the best thing to do because there was ammonia in the tank. As soon as I fed them they perked up, but I noticed a growth on my bettas cheek, it looked like when a hamster has lots of food in its mouth. As the day went on the growth grew and developed a white spot so I dosed some medicine for bacterial and fungal infections. Today the growth is still there but it isn't distressing my betta it seems. All the fish are eating fine but my platy is still skinny and clamped. I predict things are getting better because the water is clear and all the test readings are normal: 0 ammonia, 0 nitrites, 35mg/l nitrates (My nitrates have always been that high). Back to the point my bettas growth is worrying me, maybe it wasn't just ammonia in the water. Heres some pics:

This was her yesterday, you can see her clamped fins and a white path under her left eye. She was staying in the dark under the filter.

This is her today. You can see the growth is starting to go over her eye. It doesn't seem to be causing her any problems but it soon might.

She has always been a strong healthy betta and survived living in a bowl for two months! Should I go to my LFS and ask for some medicine or something or will it go my itself?

Thanks!