Internal overflow system , which is connected to a sump . What is the 15 - 20cm you mentioned ? This type of tank is usually being used for marine tank but also can be used for planted too ! The internal portion of the Internal Overflow System , IOS , do take up some real estate ie , space in the tank so will affect your planted tank aquascaping so be aware that you will have lesser space to aquascape compared to a normal rectangular tank without an ios portion .
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