8 tablesoons (24 teaspoons) for 20 gallons, is a safe dose for most aquarium fish, even recommended by some people (aquarium salt sellers) for all tank, but not nearly enough to effectively treat ich. I'd guess disease killed your fish. Do you have a heater in the 20g? Ich thrives in cool water, so you have to careful with the temp of large water changes too. Big water changes are actually helpful against ich (you get some the disease organism out of the tank) and alway good when you have dead fish polluting the water, so I don't think that hurt. That salt that you used was reduced to 38% of what you started with by your water changes anyway. If you see ich again, find explicit 'salt cure' instructions on this board or someplace else and follow them exactly. After a death, don't add fish for 3-4 weeks after you think the tank is 'cured'. Otherwise, you will just get more deaths. There is nothing disease likes more then a newly arrived stressed fish.
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