Problems cycling tank

I started with three very friendly goldfish and now I've got two left
that don't look good at all. I thought adding plants to the tank
while it cycled made it more stable. What's going on???
Setup is 20 gallon tall with 1 watt/gallon light, and a whisper
filter.
The plants a moneywort, carolina wort, sagitarius, amazon sword and
walmart bulbs (one of which has sprouted after 1 week)
The tank has been running for almost three weeks. I'm planning on
testing pH and nitrates tomorrow to see what's going on.
Should I remove the plants?
I also have an undergravel filter powered by a powerhead that I can
run. Should I get this running?
Thanks
Heather