I recently got a couple of new discus and they developed velvet disease soon after being put in my tank. I treated the tank and the velvet seems to have gone. One of my new discus hid the whole time and didn't eat, even after the velvet was gone. A few days after the velvet was gone i noticed a white spot on its side and it looked like there was white fluff coming out of it. This spot grew rapidly larger and opened up into a large sore. I have searched the internet extensively and haven't been able to figure out what this sore is and my discus is quickly deteriorating. I've moved it to a small quarantine tank and am currently treating it with protozin. I put some protozin directly on the sore as well and this took the white colour away from it. Currently the discus is lying on its side on the gravel and i can see some discolouration spreading across to its other side as well. If any body has any idea as to what disease this might be, it would be greatly appreciated.
I've attached some pictures, the first picture shows the white spot one day after i first saw it. The second picture shows the sore two days after the first photo and the third picture is a close up on the same day.