Still having PH problems

I was having Ph issues while my girls were in the 16 oz hempy cups because I didn't have Ph meter. Now I have a meter and decided to transplant the girls into bigger buckets last Sunday. I fed them PhD nutes at 5.7. Yesterday I decided to flush all the girls with 5.6 PhD water. Today I came home and decided to check the Ph of the water in the hempy bucket Res. The damn Ph had risen to 6.7. What in the hell is the problem and what do I do. Using 100% perlite. PPM 700. Girls are yellowing from the bottom up like its a nitrogen deficiency. Girls will be 5 weeks old from seed tomorrow.

That's just how hydro works 1 min your at 5.8 and 10 min later your at 6.3 it Never stays at a stedy number. That's just how it is u have to always keep a eye on your ph. So wat I use to do was put my ph at mybe 5.4 and let it rise to 6.5 b4 I would ajust it back 2 5.4 and I bout a constant ph monitor. Thats the beauty of soil growing just water, ferts and go. Hydro growing is like Driving a soup up sports car, u get where u wanna go fast but if u take your eyes off the road U CRASH. So just be very vigulent and check it every so often and invest in a constant ph monitor.

85 freaking views and no answers. Come on guys please help!!!

what size rez are you using?

Did you treat the perlite before use? what method are you using? how big is your res? what nutes and combinations are you using?