If you have yellowing lower leaves, your plant isn't getting enough N. I don't know if I would use that high phosphorus bloom fert. I think that kind of thing you just use one at the beginning of flower cycle and not again. Sometimes pH rising alot in hydro. I did a DWC once and I went to bed after lowering the ph to 5.8 and it was 6.7 in the morning. I couldn't figure it out but came to the conclusion that fighting it was a waste of time. I'd focus a little more on the nutrient issue, making sure they get enough ppm and it makes sense that if you run the cycle longer or twice every 15 minutes, this might actually solve the yellowing without adding much in the way of fert. If the plants are really hungry they'll take up the fert rapidly and this will also cause your pH to rise as a rising pH usually means lower nutes in the water. I don't understand how your ppm can slightly raise is you don't add any.