How do I clean a plastic water tank?

I have a 1000 litre water reservoir / tank for rain water and it has accumulated dirt and gunk stuck to the inside of the tank. I want to clean the inside without damaging the plastic or leaving toxic residues on the plastic - this because I would like to use the water as drinking water in drought situations and when the power is out (I live in South Africa!) and there is no water pumped from the municipal reservoirs. The tank was a gift from the previous home owner but he used to collect bath water, etc. to water the garden. I wish to use it for drinking water. Please advise.

Cathy Joubert

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