help. yellow leaves.. ph problems?

this is my second grow.

bottom leaves of girls are going completly yellow. I've tried giving them fertilizer but it hasn't helped..

I'm thinking it might be the pH. How would I alter that? I've heard of dolomite lime?

They're nearly 2 months veging. Son T Agro 400W HPS

Have just shown female pre-flowers so I don't want to lose them!

if it's only the bottom leaves going yellow I wouldn't be too concerned. they are older leaves so they wilt off sooner. later in flowering, you might see leaves yellowing everywhere, which isn't necessarily ideal but it's common and not a threat.
you can get a pH test kit from specialty stores or online. it treats the water and regulates the pH of the soil.