New Tank Still Struggling??

Hello All,
I posted here a few months ago when I was starting my first tank (I
guess I still am starting . . ). Here's a synopsis.

Started with a 26 gallon tank And added 2 Mollys, 1 Guppy and 1
Swordtail. Not fully understaning how to cycle the tank, all but one
of the Mollys died after two weeks due to the ammonia spike. I kept
monitoring the ammonia levels and after a few weeks, noticed them
dropping off and the Nitrites spiking (as expected ). A few weeks
later the Nitrites started dropping and the Nitrates started climbing.

All the while, I was doing regular partial water changes (about 20 %),
in the hopes of saving my one Molly, even though I knew it would extend
the time it would take to cycle.

After a week or so of no noticable Ammonia or Nitrites and a small
amout of Nitrates, I added two Female Plattys. I monitored my levels
for about two weeks and everything looked normal (Ammonia=0,
Nitrites=0, Nitrates minimal).

This puts me at this past weekend

I noticed my Filter (Whisper Power 30) overflowing from the 'Wonder
Tube Chamber', indicating the need to change the filter (according to
the manual). I purchased a new filter, did a partial water change
(20%) and added 2 new fish (1 Swordtail, 1 Molly).

Over the last few days, I've been checking my levels and have not seen
any unusual spikes in Ammonia or Nitrites or Nitrates, yet by this
morning, the two new fish I just purchased were dead (after 4 days).
Another Check of the levels shows the following:
pH: 7.8
Alkalinity: 120
Hardness: 7.5
Ammonia: 0.25 mg/L (hard to tell by the color result, but appears to
be a trace)
Nitrites: 0 mg/L
Nitrates: 20 mg/L

My biggest question is if changing the filter was a bad move as there
appears to be a trace of ammonia. I wouldn't think that this would be
enough to kill them, however, and that there should be enough bacteria
in the bio-foam and gravel to handle the load.

My other three fish, including the two I bought a few weeks ago seem
fine, at least for now.

Any help or suggestions would be appreciated.

Jon