I make a fresh batch of nutrients & adjust my ph to the level I need. A day or 2 later I check my ph and it is way out of acceptable range. What causes this & how do I fix it?

Ph fluctuation is usually caused by the reservoir being to small. It can also be caused by the nutrients settling to the bottom of the reservoir (poor water & oxygen circulation in the reservoir). If the plants have used a lot of water in that 1 or 2 days then it could be from the water usage. If the quality of the water is poor (a lot of minerals & deposits in the water) the ph could change very fast, the only way to fix poor water quality is to get a good quality water filtration system or buy distilled water.

Nutrients settled on bottom of reservoir.