BY the time you are ready to make new tea, your old tea has turned to mostly protozoa. Adding a cup to jump start a new brew would work in a pinch, but you will get no new fungi spores or nematodes. Essentially you lose diversity, which is the key to fighting disease. Ideally, we want the tea to unfold the same way each time and adding in old tea throws off the balance.
PH rising from 5.6 to 6.0 in 24 hours sounds normal depending on your number of plants sharing the res and their stage of growth. There is no such thing as a perfect PH level. There is a perfect PH range. In a DWC, it is desirable to set the PH to around 5.5 and let it drift upwards to 6.3ish before adjusting again.
I use liquid light + saturator + snow storm every third day. When the cuttings are young they get several foliar sprays with tea.