It is important to remember that pH naturally fluctuates as the plants consume both water and
nutrients at various rates over their lifecycles; consumption of water typically lowers the pH as nutrients concentrate in the solution, while consumption of nutrients typically raises the pH as the solution becomes more dilute.
In your case, since the
nutrient solution consistently increases from 5.8 to 6+ on a daily basis, it sounds like your pH adjusting additive doesn't have a long buffering capacity (i.e. can't hold a solution at a desired pH for a long duration of time).
I wouldn't worry too much about it. I would only become concerned if the pH leaves the appropriate range for adequate plant growth. There is an old saying that "day traders have more heart attacks than portfolio managers," because by definition they are constantly watching the stock market fluctuations whereas portfolio managers look for longer trends; looking at the data constantly constantly can cause you headaches because sometimes the noise (natural pH fluctuations) cannot easily be distinguished from the trends (pH stability).
In other words, I wouldn't be concerned on a daily basis about the pH - it will drive you crazy trying to stabilize it, and worse, by manipulating the solution with too many additives may give you short-term victories (pH stability for a day or two) while building towards long-term problems (e.g. toxic solution or nutrient lockout).
There is also a recent thread on pH down fluctuations you might want to check out as well...