This is due to the relationship of Alkalinity and copper. Copper ions become inactive when the alkalinity is above 250. A good range of Alkalinity level is between 100 - 180 mg/L (ppm) and within that range any level of copper works very well. Alkalinity levels in water features with high evaporation rates can be elevated and continue to elevate as the minerals are left behind and additional minerals are added within the new water used to top-off the water feature. Too high of alkalinity may be a reason why you don’t see results with ionization or had good results, but then notice it decline later in the season. One easy way to reduce alkalinity if it gets too high is to conduct a partial water change.