The absorption in liquid water is minimal in the visible range and even in the lower infrared or upper ultraviolet. There are also many other various particulates in seawater but I'll ignore there negligible effect. The greater requirement for the reef tank is to ensure that the depth of the tank is accounted for to ensure sufficient penetration to the bottom (depends also on what is in the tank and where in the tank). This is accomplished with higher power LEDs, or putting optics on them to adjust the angle of the LED beam (typically about 90 degrees).. A wider beam for more coverage or closer to the water positioning in a shallower tank. Narrower for more penetration in a deeper tank or more coverage via greater distance from the water placement.

Liquid water absorption spectrum across a wide wavelength range.

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