I am unsure as to whether or not the metal that moves within said field must be ferromagnetic.
Yes. Your imagination is 100% correct. When any metal moves through a magnetic field, the electrical energy will be produced.
Go slowly when making water harder. You can add buffering salts to bring it up. You may have to go to an aquarium specialty store not just Petsmart. Its likely out of their knowledge range. Kent Marine products work for me, I use them to buff up reverse osmosis water which is completely void of any minerals and is totally soft. Dolamite in the filter can also work. Avoid Quick Lime, just plain dolamite, you can get it at Home Depot or Lowes in the garden section. Good luck.
For carbonate hardness (ph buffering), use Seachem Malwai Buffer or Baking Soda. For General hardness (extra minerals), just use epson salts from the pharmacy.
It needn't be ferromagnetic. The changing magnetic field will produce an electrical current in the copper. Feromagnetism is the metal being brought near the magnetic field taking on a strong magnetic field.
Copper works better being a better conductor. We are dealing with induction here rather than just magnetism. Moving wire (any material) + magnetic field makes electricity.
