Do not add an airstone. It will agitate out the CO2 in the water like shaking a soda removes the fizz. Then plants will grow slower as they will have less CO2 to use.
Again, pearling is NOT a direct sign of good plant health. It is a reflection of a dozen water parameters that do not affect plant growth very much.
If you want to see pearling again then buy a pressurized CO2 system so the plant's metabolism increases and they make more O2, raise the water temperature to 82 F to increase metabolism and decrease the capacity of your water to hold oxygen and finally ensure that the plants do not get nutrient deficiencies or this will reduce growth rate and the amount of new tissue that can produce oxygen.
Excel can act as a toxin when too much is dosed. Certain plants are more sensitive than others. This may reduce the rate of oxygen production.