There is only one Cleanse Tag in Pokémon Platinum, and you’ll get it from one of the elderly women at the top floor of the Lost Tower.
You’ll have to clear the Lost Tower’s top floor and get rid of the fog by using Defog. Then you just speak with the woman on the right and she’ll give you the Cleanse Tag.
You can find the Lost Tower just south of Solaceon Town.
The Cleanse Tag is a Held Item where if you give it to the first Pokémon in your party to hold, wild Pokémon encounters will be reduced by one-third.
Cleanse Tag Location (Step-by-Step)
The Cleanse Tag is obtained at the top floor of the Lost Tower on Route 209, a short distance south of Solaceon Town.
Defog is required to receive the Cleanse Tag, so make sure you have a Pokémon in your party that knows the move.
When you’re ready, head south from Solaceon Town’s Pokémon Center. You’ll quickly exit the town onto Route 209.
The Lost Tower is the obelisk structure you will see on your right.
So head down the stairs and enter through the front entrance.
There’s nothing to see on the first floor, so just head for the staircase in the northwest corner of the room.
The second floor only has one Youngster trainer on it, so either battle or avoid him. Then climb the only staircase you can take (on the northwestern side of the room).
On the third floor, the Lost Tower becomes choked with a thick fog.
This will make battles much harder for both parties, since fog reduces the accuracy of moves to 3/5th of their normal stats.
Use Defog here to reduce the amount of time individual battles take to finish.
After the fog has cleared, head through the room in an approximate “U” shape to get around all the gravestones and reach the next staircase in the northwest corner.
Both trainer battles here are optional.
Once you’re up the stairs, use Defog on the fourth floor as well.
There are five trainers here—one for a single battle and two for double battles—so pick whichever route you want to take and head to the next staircase.
There are no battles to fight on the fifth and top floor – just two elderly NPCs standing beside each other at a pair of gravestones.
Both are distraught about the fog up on this floor, so use Defog to ease their anxiety.
After the fog is cleared, you can speak to the woman on the right to get the Cleanse Tag. You can also speak to the woman on the left to receive the Spell Tag item.
Note: This is the only available Cleanse Tag available in the game.
What Does the Cleanse Tag Do?
When the first Pokémon in a party holds the Cleanse Tag, wild Pokémon battles are reduced by 1/3rd. This works even if the Pokémon holding the Cleanse Tag has fainted.
The Cleanse Tag is useful during general exploration, such as when you are being swarmed by wild Pokémon in a cave and are low on Repels.
But as the game progresses and you start accumulating enough money to keep well-stocked with Repels and Escape Ropes, the Cleanse Tag’s meager random encounter decrease isn’t worth using to take up a Hold Item slot in your party.