I catch my runoff in a pan. I collect that water with a turkey baster. I water until I get enough runoff to fill 1 1/2 solo cups. I discard the first full solo cup and test for my PPM’s and PH in the half full solo cup.
For ph down I use general hydroponics PH down.
Yes, you’ll want a humidifier if your RH is that low. You can put a bowl of water in your tent and see if that will raise your RH enough.