Metals may not be visible when you first fill your spa or pool but they are commonly found in the water.
Removing them is necessary to protect your surfaces & equipment, and keep your water from becoming discolored.
You should remove the metals in your water before adding sanitizer. Once you add sanitizer to your water this causes a chemical process called oxidation. The metals will fall out of solution causing staining on your spa surfaces or pool liner which can lead to discoloration within the water.
To prevent this from occurring, it is recommended to use a Pleatco Pre-Filter, attached on the end of the hose while filling up the hot tub or pool. It is also recommended to use Unique, which is a chemical that is meant to take out metals from the water.