What do you use to clean your waste tank?

Oldgirl and Staffy

I'd welcome advice on what fellow caravanners use to clean/neutralise their waste water tanks.   In the past I've  just used bicarbonate of soda mixed with relatively hot water in the tank and left it to work before flushing and draining down.  This year
that hasn't been quite so effective and I wonder whether it needs something stronger.