Athosanswered grow question 6 years ago Perhaps, although it might not be due that. Keep in mind that as plants aborb more water (or more nutes) the pH balance of you reservoir will change as the concentration of nutrients dissolved in the water changes, pH instability can be a sign of a reservoir too small for your need, since a small water (or nutrient) uptake drastically changes the concentration of the solution. good hydroponic nutrients are buffered to prevent this. What you can try is to buffer the solution yourself by adding more pH DOWN AND pH UP, to force (buffer) the solution to stay stable at the desired range.