bump the temp up to about 86 degrees, treat with an antibiotic to prevent any secondary bacterial infections and water changes using a gravel vac to siphon debris out of the substrate.
What you see on the fish is the end stage of their life cycle is actually an egg cyst. When it bursts all the trophonts (babies) go down into the gravel where they mature for a while until big enough and then they are free swimming and find a fish to attach to and it all starts over. By using a gravel vac daily you remove some of them from the gravel bed.

That is why one day you see a few spots and then suddenly they are gone and then 3-4 days later a major outbreak.
First sign is always labored breathing as the gills are the first place the ich collects as the fish breath.
By the time you see spots, you're tank is infected and assume all the fish are carrying ich, so action needs to be taken.
For now, good housekeeping, water changes, increase temp (it kills ich) and treat with antibiotics.