On F/W W/d's it's the center section refered to as the sump, separated on each side by baffles. Plants are inserted here either in pots , floating (ie water hyacinth or water lettuce) or rooted in a medium sand base. Used to remove excess ammonia / nitrited / nitrates that the bio-filter doesn't get first time around. Do not use if you have aplanted tank.
Dr Joe
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