They will need some decent sized heatsinks. I would stay away fromt he CPU ones because they are very dependent on the heatsink and fan being dust free to cool properly, and the fan tends to suck dust in. Besides the constant maintenance there's the chance of unclean dust getting into your tank and I know some of the corals and stuff are pretty sensitive.
I would go with a decent slab of aluminium bar, like a 4"x3/8 profile (100mm x 10mm) and the full 6 foot length of the aquarium. That will provide plenty of heatsinking but is shiny, anodised silver easy to clean and no cavities or fins etc to collect contaminants. Or maybe even hang 2 bars side by side. It's not that expensive, maybe $20 for a 6 foot length.
Of course if you are mainly using the smaller LEDs (like 1w, 3w etc) you could get away with thinner cheaper bar, they sell 2" x 3/8" and i think a 3" width too (80mm x 10mm). Check your yellow pages for "aluminum suppliers" and they will usually saw cut it for you while you wait.