Is it in the tank by itself? Do you have the ability to do a full tank water treatment?
If so, the trick will be nitrofurazone and kanamycin mix. Use at 2/1 ratio of the drugs. That tank would be about 4 grams nitro and 2 grams kana. Just add extra airstones and pull out any kind of chemical filtration you are running on the tank. It will probably take 2 weeks or so to see a turn around once it's this bad, but you can get it back. Just keep an eye on ammonia and do 50% water changes as needed. Redose the drugs for the amount of water you are adding (IE: if you do a 50% change add 2 grams/1 gram to make up what you took out.)
If so, the trick will be nitrofurazone and kanamycin mix. Use at 2/1 ratio of the drugs. That tank would be about 4 grams nitro and 2 grams kana. Just add extra airstones and pull out any kind of chemical filtration you are running on the tank. It will probably take 2 weeks or so to see a turn around once it's this bad, but you can get it back. Just keep an eye on ammonia and do 50% water changes as needed. Redose the drugs for the amount of water you are adding (IE: if you do a 50% change add 2 grams/1 gram to make up what you took out.)