I suggest putting your surving fish in clean water in a separate tank.
Then clean your old tank. Use a 10/1 bleach mixture on all of it,
especially the rocks - everything you want in your tank, including your
pump, lines, etc. Put a couple drops of food coloring in the tank with
the bleach mix - red, blue, or another bright color.Let the bleach sit
in the tank for about 20 min to kill everything that might be growing
in there.Throw out your living plants (sorry) Then rinse the heck out
of the tank - over and over, until there is absolutely no trace of food
coloring left anywhere, and until you can't smell the bleach.Replace
your lines and filter pads, make sure everything is rinsed clean, fill
with fresh water. That way infections and chemical imbalances are
eliminated. Your fish will have the great stress of the new tank, and
the lack of nitrate/nitrite converting bacteria will exist, but both of
those will pass.

Good luck!