So I got this discus as a donation about 18 days ago. It came in malnourished, stunted and probably infected with parasites.
Upon arrival, it was eating (not a lot, but a decent amount) and I treated it with metronidazole (1g per 20 gallons every day, for 12 days) and fembendazol (2mg/L) once a week (applied twice so far)
It started getting better and better, and eating more, but as of 2 days ago it stopped eating and stated to stay at the top of the tank. Today it seems worse and isn't standing up right.
It's the only fish in the tank, apart from one 1" bristlenose pleco I keep to eat leftover food, and I've been doing 50% water changes every other day. Parameters have remained stable too.
Does anyone know what could be wrong with my discus?

Here is a video of it:

https://youtu.be/SwS0IQ48oUE