@Shippers , 10% hydrochloric acid will clear this [readily available on eBay] But do be careful with HCL, on skin and in eyes and especially the fumes...so wear correct PPE. Pour in 5ltrs and leave to work, seal all outlets and give the tank a swirl/leave upside down and swirl some more. Once gunge free, empty and rinse with warm water. When rinsing be very careful with the fumes and waste gunge...both will eat skin.
Think ill pickup some acidico at builders merchant tomorrow...we use it here for cleaning showerheads but can buy 5l for brick cleaning...thanks...