How do I treat ich in my aquarium?

Short answer

In depth

Ich (Ichthyophthirius multifiliis in freshwater, Cryptocaryon irritans in saltwater) is the most common fish disease in the hobby. Caught early it's treatable. Caught late or treated improperly it can wipe out a tank.

Freshwater ich treatment

Identify it: white spots that look like grains of sugar on body and fins. Fish may flash (rub on objects). Behavior changes - clamped fins, hiding, refusal to eat.
Raise temperature gradually to 86°F (30°C). Heat speeds the parasite's life cycle - the medication only kills the free-swimming theront stage, so you need the parasite to drop off the fish faster.
Add medication: ich-x, ich attack, or a copper-based treatment (Cupramine, Copper Power) per package directions. NEVER mix ich-x with copper.
Treat for 10 days minimum, even after spots disappear. The parasite has a 7-21 day life cycle and will return if you stop early.
Reduce stocking and improve water quality: ich rarely kills healthy fish in clean water. Stress is the trigger.

Saltwater ich treatment (the real fight)

Saltwater ich is harder. The parasite has a longer life cycle (up to 76 days from spot to spot) and is more resistant to over-the-counter "reef-safe" medications.

Move ALL fish to a hospital tank. Reef-safe medications don't kill ich reliably. Copper does, but copper kills inverts and damages live rock - it cannot go in your display.
Hospital tank setup: bare-bottom, sponge filter (cycled in advance), heater, no carbon. Match display tank salinity and temperature within 0.001 SG and 1°F.
Dose Cupramine to 0.40-0.50 ppm over 24-48 hours. Test daily with a Hanna or Salifert copper test kit - copper degrades in saltwater and you need to dose to maintain.
Treat for 14-30 days, depending on the protocol you follow. Hyposalinity (1.009 SG) is an alternative for fish that don't tolerate copper (some sharks, rays, mandarins) but requires perfect specific gravity discipline.
Leave the display tank FISHLESS for 76 days. Without a host, the parasite cycle dies out. Inverts, coral, and snails are immune. Your display can run normally minus fish for 76 days, then your now-quarantined fish can be returned.

Prevention is the only reliable approach

Quarantine every new fish for 4-6 weeks in a separate tank with prophylactic treatment (copper for marine, salt+temperature for freshwater). The hobbyist who skips quarantine because "the fish looks healthy" is the same hobbyist who loses a $400 yellow tang to ich 21 days later.