Am I treating Ich correctly?

I'm reviving a long neglected planted aquarium that was overgrown with ugly
plants and algae, and had a few guppies in it. I started cleaning it up
and doing water changes, and when if wasn't all that nasty anymore I bought
a few feeder goldfish. That was a week or 2 ago, and it's looking *much*
better. But I noticed yesterday that one of the fish was not very active,
and the others were scratching themselves in the sand and against the rocks.

I examined the fish and saw that one of them was covered with white dots.
Ick. I dipped out a quart of the tank water and added 2 Tbsp. pickling
salt and stirred until it was dissolved. I took all the fish out one at a
time and dropped them in the salt water for about a minute or until they
fainted, whichever was first. Then put them back in the tank. I added 2
tsp. malachite green (it's a 20 gallon aquarium.) The fish look better
already, and it's only been an hour.

How often do I need to retreat with malachite green? The tank is rather
brightly lit, if that makes a difference. I don't want to add salt to the
tank because of the plants.

BTW, I have a Vortex diatom filter around here somewhere if that would be
useful.

Thanks, and best regards,
Bob