Hi to all
I was given a blue discus about 4inch size.When i got it, she had rotting fins, thin and dark in color. Fins are not clamped. I put her in a quarantine tank with cycled filter, aeration and heater. Water parameters are Ph 6.8, Ammo 0, Nitrite 0 and Nitrate 20. She is eating but slow to take the food and always hiding in a corner. This is what happened:

Week 1. Applied salt in water at 1tbsp/3 gallons. Heat slowly increased to 31C. Water changed 25% daily, salt replaced. Eating a little bit and poop is dark color. 7 days salt treatment. No improvement at all.
Week 2. Treated with eSha Hexamita for 3 days then replaced water daily for the rest of the week. Still dark, eating and hiding. Not much improvement.
Week 3. Applied salt again but stronger 1tbsp/2 gallons and treated with 400mg metronidazole/30 liters at 31C. Metro dosing applied daily for 7 days.
Note: After the 1st dose of metro, it showed a nice blue color and not dark anymore but that only lasted for a few hours then back to its dark color by lunchtime and hiding.

I have stopped Metro now slowly clearing the salt in the water. My fish still dark, rotting did not progress further, hiding and eating small amount of pellets.

Please advise as i am lost as what to do.