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Hex tanks are not the greatest idea for marine aquaria. Being taller rather than longer, they do not offer enough water surface area for proper gas exchange and often lead to poor O2
levels and faultering ph
. Many marine fish also prefer horizontal rather than vertical movement.
The advice you were given about the rectangle tank is better for a few reasons. A longer tank will provide for much better surface gas exchange, allow for less restrictions in fish types, be easier to accomidate equipment and will also spread out the weight of the tank. A hex tank being tall, will focus the weight in one area. The longer tank will spread that weight better along the joyces and hopefully prevent flooring issues.