New Tank Question

I've had 3 small fantails for about 2 years in a small Eclipse tank (~2
gallons). They've thrived with no problems, but, of course, they needed more
room. So I started a new 10 gallon for them. Setup, filled, treated, etc.
and waited for 2 days (I know, I should've waited a week or so). Then I made
another mistake by adding a black and a calico moore after about a week. The
original fish were kind of timid and tended to lay near the bottom in the
back corner (but they've always been kind of timid, so I didn't think much
of it). The calico moore died after 2 days. The black moore died the
following day. One of my original fish died a few days later. Checked
ammonia and nitrate/nitrite levels.........looks good. The pH is a little
low, around 6. Added salt as a general tonic. Now the 2 fish are still
acting a little unusual, and I've noticed a few white spots/bumps along the
tailfin of one of them. Doesn't look quite like ich (white spots are less
defined), but could be. I want to start partial water changes and possibly
treat with Quick Cure, but I'm not sure if it's too early in the nitrogen
cycle to risk depleting or killing off beneficial bacteria, which could lead
to more problems. And I still can't figure out why the others died so
quickly and unexpectedly, since ammonia/nitate levels were almost zero. Any
thoughts or suggestions?

Milky-G