The cycle is done and aside from a few very dear Tiger barbs that have Dropsy and are in a hospital tank, the 20 gallon is doing well.
I think finally figuring out what it was got me to the conclusion of isolating sick fish. Since coming in contact with the sick fish's body and poop can spread it I've been doing extra gravel vaccings.
No other fish have gotten sick and all the water parameters are good.
Thanks for the help everyone. It's much appreciated.