The round "leaves" are the plant´s reservatory, they go yellow as the reservatory is over, anyway you can feed the plant, the round leaves will stay green.
I´ve seen that in a hydroponic DWC system, plants that still have that reservatory doesn´t suffer at water changes, but plants without that reservatory tends to start going down pretty fast as they got out of the nutrient solution.
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In this pics you can see(i hope so) that the reservatory is already used