pearling is an effect in which the tank water is already saturated with O2 and any O2 produced by the plants cannot dissolve into the water and hence appear as O2 bubbles. The plants will always produce O2 as long as they are photosynthesising (presence of light and adequate mix of nutrients, including CO2) but usually it will not pearl if the amount of O2 produced is not enough to supersaturate the O2-dissolving capacity of the water.
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