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[Aquattro]

Depends on the ccanopy design and the light size and how many. 2x 175w wouldn't add heat. 3 x 400w will generate a lot of heat. If the canopy is vented, it holds less than a sealed canopy. So no real answer to your question.
In my canopy, I have 2 400w bulbs and a fan. In the summer, my tank goes from 80 to 89 every day due to lights.