Ph-problems?

When I run hydro I stick with my basics, no need for extra additives. Mostly it's just important to keep your tank clean. It's not uncommon at all for your pH to change in the first 24 hours. Without going into a long explanation, it's because your introducing more oxygen to your water so your lowering the CO2 levels. Temp in your reservoir is a factor too. These things affect pH. I will generally adjust my pH a few hours after a tank change to accommodate this though you could also try to recreate your reservoir conditions when you mix a fresh batch.