Your Discus was in very bad shape when it died.
It looked like it was sick for some time before it died.
I've seen in many fish stores where the Discus are skinny and dark in colour.
I suspect they have disease either internal worms/ parasites(if the fish is skinny) or gill flukes(if they are breathing fast).
I heard that the fish will also produce a lot of mucous in the gills if they are infected with gill flukes.
This make it hard for the medication to penetrate and reach the gills where the flukes are.
Formalin should be effective in right dosage but cannot be used for long.
But I prefer to use
Wormer Plus which contains Flubendazole.
You have to use it for at least 1.5 month or longer to kill the flukes in eggs form that cannot be killed by Formalin or any medications.
Discus Wormer Plus Now in smart new updated packaging to celebrate 15 years since wormer plus first came out. Many fish come from massive river systems where water changes all the time. This is the difference to keeping them trapped in a 'glass box' & with nowhere to escape, unwanted pest can...
jzxonline.com
More info on treating gill flukes which are very common among Discus.
Flukes are a parasite. There are two main families of flukes affecting discus. Gyrodactylidea is a fluke which usually attacks the skin. Dactylogyridea family of Flukes is an egg laying fluke and live mainly on the gills.
www.simplydiscus.com
My conclusions after keeping the fish myself and many hours of reading and gathering information from Discus websites, books and forums are:
1)Many Discus are infected with internal worms /parasites when they are in the fish stores due to their feeding with worms and beef hearts. Some are infected while in the fish farms.
2)Many Discus are infected with gill flukes while still in the fish farms. The fish won't show much symptoms during the first 2-3 weeks in your tank. Some only showed symptoms after 1-1.5mths when they are at critical stage with symptom of fast breathing.
Some Discus breeders even openly declared that they can't guarantee that their Discus are free from gill flukes.
By the way, just for precaution, is your well water clean?
Is there any contamination of bacteria and parasites?