Hi everybody!

I've had my black moor fish for three months now, and she's become very sick over the past few days... She had some health problems before (she was going cloudy all over, it wasn't a fungal infection though, and her fins were becoming clamped and deformed) once I realised this was due to the tank being far too small and the water being changed too frequently I moved her to a much larger tank, put some fresh plants in and have been closely monitoring the water ever since. She seemed to be getting better, she was much more lively and her fins went back to normal, and the cloudiness on her body disappeared. She looked like a healthy fish again.

But for a few days now she's been lying on the bottom of the tank and only moving at feeding times. Her fins are extremely clamped and close to the body, and she's lost so much weight, she no longer has that healthy round-bellied black moor shape and her gills are now very open and visible, it's like she's having trouble breathing

I've just checked the water and the levels read as follows:

NO3: 40
NO2: 0.5
Ph: 7.5
Kh: 240
Gh: 180

These levels all seem normal to me, I have one other black moor in the tank and he's absolutely thriving, he honestly looks like the perfect black moor specimen. I have a filter in the tank, I put in a little aquarium salt every two weeks, and I also have three fresh plants in there. I feed my fish flake food once a day and I've been advised to do a 25% water change once a week. My tank is next to a window so it's prone to getting brown algae, but I wipe that away regularly.

I really don't know what to do, I'd be so so grateful if somebody could help, any suggestions are appreciated! Thankyou!

EDIT: I have now attached a photo which may help!