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Yes, MHs do run hot, especially if you have double ended bulbs.
What you saw was most likely the ballasts (transformers) for the MHs. Most likely there is also one or more ballasts for the actinics.
Before you worry about fans, I would first replace all the ballasts with electronic ballasts. Your savings in power will more than make up for the costs of the ballasts, The fixture will also run cooler.
As for fans there are a couple of options here. If you use the typical computer fans, they usually run on 12V DC, so you'll need a power source for them too. You can also get similar fans the run on 110V AC and wire them so they are supplies from the power cord.
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