If your faucet aerator gets repeatedly clogged with tiny blue or green granules. it's aluminum hydroxide: a corrosion by-product caused when household water with a high pH level comes in contact with the aluminum anode in your hot water heater. A chemical reaction causes high amounts of aluminum hydroxide to form on the anode, which breaks off in little chunks and winds up on the bottom of the tank. It ultimately courses through water pipes until it shows up on your aerator filter screen as tiny gray, blue or green granules that clog it up and eventually cut off the flow. To correct this, you just need to switch the aluminum anode rod for one made of magnesium. To determine which type you have, look on top of the water heater tank. You'll see a large hex nut head. If it's smooth and flat, it's aluminium; a raised bump and a welded bead says it's magnesium. If needed, replacement anodes are widely available wherever plumbing supplies are sold.