The topic at hand is about dealing with leakage from a pouch's built-in vent or charcoal filter, which can cause stains on undershirts. The person experiencing this issue is looking for ways to prevent or eliminate these stains and is curious if their habit of rinsing the pouch with water might be contributing to the problem. Here are some helpful tips and insights:

1. Avoid creating extra holes in the pouch yourself. It's best to use a pouch with a manufacturer-installed vent or filter. If you've been making your own vent, it's time to switch to a pouch with an integrated charcoal filter.

2. If the integrated filter tends to get wet and leak, consider using a pouch without a filter. Many people have found that going "filterless" helps prevent shirt staining.

3. Most pouches with filters come with small white circular stickers. Use these stickers to cover the vent:
- When the filter becomes damp.
- Before activities like sleeping, showering, or swimming, when output might reach the vent.

4. Instead of the provided stickers, you can use thin, easily removable tape like 3M Micropore. For a more complete seal, stronger tape such as duct tape can be used to block moisture, odor, and seepage.

5. Some companies offer stick-on charcoal filter patches, but they may not work well for everyone. Tape or the provided stickers are generally more reliable.

6. For clothing care:
- Pre-treat stains with a product like Whisk Away.
- Oxi-Clean, now available in a spray bottle, foams on contact and effectively removes stains.
- Greased Lightning is another option that offers similar stain-removal performance.

These tips should help manage the issue of leakage and keep your clothing stain-free.