If your fish are scratching, most probably it's due to skin flukes or gill flukes. Flukes are parasites and this infection is deadly.
Did you fish had fast breathing before it died? Then it's most likely due to gill flukes.
You may have to treat your tank for at least 1 month.
I have been treating my tank for gill flukes for 2 months.
Whenever I stop the treatment(thinking that I have eradicated them), the flukes will come back and some fish will die again.
I have many fish died in the past due to gill flukes.
The two symptoms are scratching and fast breathing.
But when the fish reached the stage of fast breathing, it might be quite late already.
You can treat it with Praziquantel to see if it works. If not, you have to use other medication.
I'm currently using Flubendazole as the flukes in my tank are immune to Praziquantel.
Currently, I don't see any more symptoms after treating for 2 months.
But I'm still monitoring.
The difficulty in treating this gill flukes is due to the eggs that cannot be destroyed by any medications.
The flukes can only be killed when they are in free swimming stage.
And the eggs can be dormant for a long time in your tank before they hatched.
This is the medication that I'm using now.
It's called Wormer Plus and its main ingredient is Flubendazole.
KUSURI WORMER PLUS, AQUATIC FISH WORMER,FLUKE & PARASITE KILLER, DISCUS WORMER, DISCUS MEDICATION, BUY KUSURI Australia, DISCUS DISEASE, DISCUS MEDICINE
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But if you prefer to restart the tank again instead of doing treatment, do let me know.
You have to sterilize many things and your filter media might need to be disposed off instead reusing it again.
I had restarted my tank a few times. Changing new tanks, filters and everything.