i went to buy a 1000 ltr ibc the other day from smiths of the dean as they are only local for me i drove over and spoke to the owner "ray smith".
He asked me what tank i required and what i wanted it for after telling him it was for storing pure water for window cleaning he said i should get a new tank as if there was any residual stuff(orange juice, food, chemicals.) stored there before, it could contaminate the water.
you may say he was just trying to sell a more expensive tank but i know this company isn't desperate to sell as they are very busy and he was very genuine not pushy, just a thought if you get spotting on windows could your water be contaminated from the tank even if it doesn't raise the tds on you meter? just a thought.
Regards
Tom