2% HCl or HNO3 should be fine. It is said that we add acid to bring the pH to 2 which requires few drops of concentrated acid (HNO3 or HCl). The idea behind adding the acid is to cease microbial activities.

As Manish says, the objective is to get the pH of the sample to

I totally agree with John Machell's and Manish Kumar's answers. With most water samples I used 1-2 ml HNO3 (preferred because all nitrates are highly soluble), depending on the buffer capacity of the water samples.

2% HNO3 or HCl

Peter Faber agreed with Peter that nitrites and nitrates are exceedingly soluble, which is why APHA recommends pH

Is there a way or method to extend the preservation of heavy metal in water using HCl?