I am using HPLC for some analysis. Feeling a lot of difficulties if the water is left over unused even a day. I am sure about it because it runs well with 100% ACN.
What Hediye meant is that depending on the detector you are using with the HPLC, you can have Acid added to your Water used as mobile phase. In my case, I am using a mass spectroscopy with the LC analyses, thus acid is very important to be added to the mobile phase in order to assist in making protonated ions in the MS.
So you can do some research on the application you are doing now, and find out if the addition on acid to the mobile phase is harmful to your analyses or not. Then you can decide to add little bit of acid (usually 1% by volume) which makes your water highly acidic and hard for bacteria to survive in.