Oh boy do they heat up. They're far more efficient than most other forms of lighting that you could use, but they still churn out heat along with the light, and the ultra bright stuff that we use in aquariums do get hot. The fixture in general will tend to be cooler per light output than other lights, but the heat points are concentrated, so local overheating is an issue, and spreading that heat with heat sinks and heat conducting material is vital. If that fails, then things go wrong, as LEDs are also quite heat sensitive compared to other options.