It should not be a concern as brass is not very soluble. And if any minute amount did leach from this fitting, it would likely precipitate out of solution....especially in the presence of an elevated alkalinity like in saltwater. Using our CupriSorb is an excellent indicator of copper as it is a color indicating resin. If you use this resin in an area of high water flow for a few hours (even up to a day) and do not experience a color change, then copper is not an issue. You mentioned that it has been in your system for 5 days with no color change. It is acceptable to leave it in your filter if you choose but not necessary.