60cm long x 30 cm wide x 45 cm deep
23" long x 12" wide x 18" deep
Oh not so true. Depend on the area of the heatsink, thermal resistance, ambient temperature. This all needs to be calculated.
Let's see, just for laughs, 24 leds at 0.4A with Vf of about 3. That's
0.4x3=1.2W per led, 29W per system. A big heatsink with a lot of fins can easily dissipate that much heat. I had NVIDIA 7700GTS passive cooled
video card wich dissipated around 120W of heat in closed space. Of course, there was air flow in the case and there were 2 heat pipes, the the space was tight and air flow is was not free. So, just a heat sink with nothingh above can feed the ambient air 30W for sure. Have you notice fans in your LED display?
However, you gave me an idea which i would not do
Pass water to an external filter through the heatsunk to collect the heat. Then it will cool while in the filter and on it's way back and will rise the water temperature a bit, so one can save on the aquarium heater power. Neat, huh?