diesel in water tank

Hello,
Newbie again.....
I actually just put £30.00 of diesel in the wrong tank.
Put it in the water tank.
I have drained it. Left drain tap open.
I have run cold water through the tank, left drain plug open.
Any ideas on how to clean water tank?
Its the mistake even I will only ever make once.
Please help, its been a miserable Sunday so far.

It's all here.....

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Thanks again,....
Brilliant link! I will probably order some toilet chemicals at the same time. So I am at the petrol station putting a bit of diesel in the van, half a tank as that will do for the winter, I have winterised the water tanks, all drain taps open as you kindly advised. I checked the fuel filler cap to see which side it was on so I would draw up conveniently for filling. So I put fuel in.......big puddle appeared........confused I saw liquid coming from under the van, I turned off a drain tap and it stopped the flow. It took a while, but it gradually dawned on me what I had done....diesel in the water tank!

when you drain the tank, put the water/diesel mix in a container and leave to settle for a week or so, without moving the container, carefully syphon from the bottom of the container any water and you will have your £30 of diesel back.

You may have to carry out the procedure a few times.

ps. it is better to leave a tank full of diesel rather than half, reduces the likelihood of condensation (water) accumulating in the diesel tank.