Gill Fluke is caused by bacteria, it is common and can cure quite easy. Your fish probably died because of extreme level of salt in the water. With 7.5tbsp/ gallon your water becomes even more salty than the water in the sea (and discus is pretty much a
freshwater fish). It is very harsh and a very risky move on your fish.
I believe that Ted's method is a salt dip. But uses extreme amount. I'm not saying his way is wrong, I just want to also share another method of performing a salt dip that I've learnt that worked for me and many others:
"2 tablespoons per gallon water. ANY salt so long as there are no anti-clog additives. Iodised salt is safe, cooking salt is cheaper.
Mix in a clean bucket with water of the same temp or from the tank. Net fish and place into bucket. After 30 mins remove back to clean water, BUT if the fish rolls over before the 30 minutes is up then remove it straight away to clean water. (Many other methods of salt dip on the internet says you should wait until the fish roll over, but IMO that is way too harsh.)
I normally give the tank a good clean through when doing this or put them into a newly setup tank, but not everyone has the tankage.
The dip can be repeated daily (even twice daily) if needed for as long as necessary (well easily 2 weeks)."
By Connie from SimplyDiscus
This is only 1 way to treat gill fluke. You can also use commercial medication which are readily available at any good fish store. You can treat them with a med called Formalin and follow instruction on the bottle. If not, pimafix is another common one and its from
API (same brand as the test kits), therefore very easy to find. But with medications, you always have to be careful and use with your own risk.
GoodLuck