Lots of clorox. Seemed to help me.
Drain the system, fill with clean water (and a full bottle of clorox), drain again, repeat. It helps to have an external pump to drain the tank - works faster and does not wear out the water pressure pump.
Also, if you can reach inside the tank - open it and clean the walls. Use flexible pressure wand (they sell those things to clean out RV toilets) to power spray with water inside.
This might take a while.