I was reading about ich, not here, but the amount I added was right in the middle of the recommended amounts. Perhaps too much, I realize now. I wonder if it even was ich. At any rate, it was the wrong thing, and too late.
Anyway, once the little guy passed, I did a total water change to get the salt out of there, lowered the temp, and am now finishing up an abx treatment. Seriously, once these little ones start to languish, it seems very difficult to help them. I sort of feel like my mistake with the salt only sped up the inevitable. That does not minimize my culpability, however.
As for the big tank, I did a really good gravel vac yesterday, and will do another today in the areas I couldn't get to. I think one of my problems is that I leave the cucumber slices in there for too long, so I'll be rectifying that. And a good cleaning of the canister filter (proper rinsing in tank water). That will have to do for now.
1 - 55 gallon planted community
3 - 10 to 20 gallon planted betta tanks
My advice: slow down, think, and be willing to learn. Then you'll be fine, no matter what.