100% water change after about 4½ months??

I have had a 10 gallon tank with 3 adult sword tail fish 2" in length
for about 4½ months.. the water is really dirty because when I did my
50% change as recommended by Tetra, my plastic bucket had some
dirt/debree in it that I didn't spot until after I added the 50%
remaining water..

My test kit shows medium water hardness, borderline Nitrate/Nitrate,
and border line unsafe PH.. I was told at my recent visit to Petco,
that I should completely drain my tank, clean all decor with water and
replace all water with filtered water and let sit for a few hours
before re-adding my fish.. Also they recommended a bubble stone to
introduce more Oxygen into my tank...

Has anyone done this before, and any tips, etc to help along the way??

PS.. My fish have had about 17 fry in the past few weeks that I have in
a breeding net...