Never use silver dip! This will cause irreversible damage to the silver and marcasite stones giving a brown, burnt appearance.
Always remove marcasite jewellery before participating in any activities involving water/liquids, such as hand washing, cleaning, bathing/showering, swimming, using jacuzzis/saunas/steam rooms etc. Most marcasites are set with water-based epoxy and this will come off if immersed in water and the stones may become loose.
Detergents, oils, perfumes, make-up and hairspray all contain chemicals and can cause tarnishing and damage.
Gently wipe jewellery clean with a soft cloth when needed.