Can someome tell me why my leaves are yellowish like this

i'm in my 5th week of flowering and a lot of my leaves changed a yellowish color .......can someone tell me why ?

Looks like a mag deficency .
sorry for the spelling ^

Looks as if you dont monitor your ph, your plants are terribly locked up Start with some hydrated lime in your water then a good feeding after the lime water drys out, you cant fix the damage but you can correct new growth

A odd question , Do you use a water softner, or is your water pulled from a well ?

I think what happen was the pump did not work for a couple of days ,,then i noticed it and today it looks like this ..do you think i should flush ? or what else could it be?