EC variations in aged water

Get on all, i recently bought an ec tester and ph reader to make sure the nute levels and ph levels were spot on for my plants, i was told by the grow shop guy to test the tap water for ec levels first and to add required nutes plus whatever the tap water reading was, ok fine i get that but i keep a large container of water to age it and reduce chlorine etc and i noticed the ec level after 3 days was 0.2 where it hadnt even registered on fresh tap water, and when i left it a further 7 days it had risen to between 0.2 and 0.4 i can only assume this is due to evaporation leaving some kind of salts in the water.
This is fine to a point as my aged water can be tested and nute levels adjusted accordingly but my feed tank contains an airator and water heater(which takes a day to bring up to temp), and my aging tank does not so evaporation is at a greater rate, this plus the fact there are remains from the previous feed/water mix mean that the ec readings could be reading higher than they are especially as it takes 7 days to use the tank up.
Appoligies for a long and probably stupid question but first time grow and all that, any suggestions welcome, cheeeeers
 
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