usually people need a little more info like what media are you using what nutes what is your ph generally stuff like that before anyone can make an assumption
Are you saying once you got your ph meter working you readjusted you ph to 6.0, if so give it time to recouperate, also soil may be too hot for your seedling.
A couple times a day is waaaaaaaaay to much! that leaf is rotting!!! common sense bro. water when you're soil feels dry, and when you're container is feeling light.