Metal rusts, causing toxins in the water that can be fatal to fish. I recently found brown discoloration on my Betta fish, DaBaDee's, Penn Plax spongebob pineapple house decoration. After removing the decoration and trying to scrub off this brown stuff, I realized that it was actually rust from exposed metal used in the internal structure of the decoration. This rust exposed my fish to toxins, causing him to become sick and clamped, lethargic, and resulting in stress and color loss. He is still recovering, but I don't know if he will make it. I can't believe companies like Penn Plax can get away with selling decorations that contain metal as safe for aquariums, and my fish has suffered because I trusted this well known brand.