I have used metal free, sequasol & Jacks Magic. They are all good. To add the fill water through the filter, you put your hose in the skimmer (with a trichlor puck). The water will hit the puck, and flow right into the filter before it is returned to the pool, so it could help the metal stain the filter instead of the pool. The sequestering agent should keep the metal in suspension - make sure you add at least enough of what the bottle says, because more won't hurt anything, but not enough will leave some metal free to fall onto the liner.