that plant looks fine to me. don't use rain water, it picks up dirt/pollution or whatever else is in the air on the way down. most ferts will buffer to a point. what are you using? don't flush! your plant isn't displaying any problems i can see. ditch that meter, and get something else. when i first started, i used the paper strips, and they worked for me. as long as you properly ph everything you put in/on your plants, ph should never be an issue. in soil, a lot of folks do a feed, feed, flush schedule.