Quick question to You Prawn.
How long , more-less does it take for Your system to go up from PH 5.8 to 6.3 ?
Is it weeks or days?
I've noticed that in my 80L tank it takes around 2-3 days to go up that much before I have to reset it. Doesn't that sound a bit odd ?
Sorry for being late to the party . . . It's the microbes. Beneficial bacteria remove CO2 during the nitrification process, rapidly raising pH. That's why pH always rises in a healthy (non-sterile) hydroponic system.
So yes, a pH swing from 5.8 to 6.3 in a matter of days is normal. You'll also notice when you test your runoff it will have a very high pH compared to the nutrient solution going in - a combination of microbial activity and your plants using up nutrient. If your runoff comes out more acidic, then that's usually a sign of anaerobic (bad) bacteria and potential root rot or other disease
While ph5.8-6.3 works best, anything from ph5-7 or above is acceptable as long as you don't leave it at either extreme for too long. You could try dropping your pH to 5.5 initially and let it drift to 6.5 if you don't wish to adjust it so frequently. I'm usually topping off my tank every few days anyway, so I expect to see my pH at around 6.3-6.4 before dropping it down to 5.7-5.8 again.
I believe pH drift is beneficial for plants as it fluctuates naturally and makes different nutrients more or less available at different levels, providing a broad feeding spectrum (so to speak).