Can lemon juice and water mess up my tank my girlfriend put it in
Maybe. How much did your EX-girlfriend pour in?
She said she poured one water bottle and a little lemon juice mixed
One water bottle. You mean a gallon bottle? One liter? 12 oz? What size?
What tank are we talking about ? I also don’t know what amount this bottle holds . And why did she do it anyway?
I am very interested in what the mechanics say to this question. My instinct is “yes.”
You could try some dry gas if it was not too much water. Otherwise you will need to pull and drain the tank.
Did you ask her why she did this, and more importantly will she do it again, and even more importantly did she pour foreign substances into any other fluid hole in your car?
Because a 12 ounce bottle of water will probably just be annoying in your tank since you have to drain it, but pouring lemon water into the brake fluid reservoir would be a whole 'nother story.