You have done everything except what would have been my first choice. Trytreating with metro for 12 days at the rate of 500 mg per 10 gallons for 12 days. Do a huge WC every day during treatment. BTW, if your nitrate goes as high as 40 you aren't doing nearly enough WC. Most folks say that 10 is good enough. I keep my Discus tanks at 5.