runoff ph way to low

Are you growing in soil? or soilless? are you feeding liquid or amendments? The PPM should not have kept aorund 1000PPM if all you where entering in is just water and gallons and gallons of it. something seems off. Good advice about calibrating your PH pen below. maybe it is not reading the PH right. you need ph calibration solutions, some say you can use distilled water but its not a great option and damages the probes tip. The same goes with your PPM pen, mine does not stray to much but I calibrate it once a grow and let it sit in the solution for a good bit and shake it all aorund to ensure it not a false low/high. Can grab some PH strips as a backup. Pool supply ones are good, and they have dual indicators to make it a bit easier to see the number. Good Luck!

Check your pH meter dude something isn't right.