Aluminium. Thickness would obviously vary on strength of magnetic field to be neutralised!
Go to your hot water tank, and check the hot side water pipe is not plugged up at the connection point to the tank. Turn off the incoming cold water to the tank --- relieve the pressure by opening any hot water valve (tub/kit.sink). Take the hot water line loose from the tank and inspect. You can inspect the cold side as well.
You need to find out what heats the water if it is tankless water heater on a gas or oil boiler there is probably a flow restrictor in line at the tankless to slow the water down to properly heat it and it may have an adjustment. If it is a gas or electric heater check the cold supply valve. If that doesn't work the nipple at the top of the heater can get scaled up and block the passage of water Good luck
Check the water shut off valve on top of the water to make sure it's all the way open (open and close it a couple of times). Check the screens on all the faucets for debris (unscrew where the water comes out). Perhaps draining the water heater for routine maintenance to clear out rust or mineral deposits. Good luck
