I want to install 3x 20W LEDs on a 1.5m x 4cm x 3mm aluminum bar. 
Is the bar large enough for 60W ?

Known:  

From literature I have found that about 2/3 of LED's power is converted into heat.  

Using thermal paste  

The heat sink will be mounted 10 cm above water (500L aquarium) - I think this will massively improve the heat dissipation.