Cleaned properly or not?
Methods, people cleaning via vacuum much as like with swimming pool suction, I don't think can get tanks cleaned as shown, and the walls maybe a problem, (apologies  with my lack of computor skills) Sediments in rainwater tanks tend to congregate in the centre of the tank, this tank  had about 150mm  high sediment in the centre  going down to about 25mm around the perimeter. I have doubts that such methods  can ever get the results shown here, please note the scum, residue on the tank walls.

My concern is primarily with the risk of sprays and known Carcinogens, many of which are KNOWN to be connected to prominent Cancers and Child Leukaemia in Western Australia.

My veiw is that drinking water Tanks should be cleaned to the best quality, not the lesser Quality, some carcinogens can cross contaminate three Generations look at the escalating Cancer within W.A.