You know, this thread is really great! I've been planning on building my own canopy for a while now, and this has inspired me to go pickup the wood and hardware tomorrow.
I must admit though, I have a certain degree of expertise when it comes to indoor plant lighting... I spent years growing plants indoors using specialized lighting and homemade reflectors. I've got to say, what you heard from your boss, beetlez, is correct... Flat white paint is very effective.
There are a few reasons to use flat white paint instead of aluminum foil or mylar. First and foremost in my opinion is that foil and mylar tend to create "hot spots". Think about it this way... When you hold your watch just right in the sun and it reflects a tiny dot of super bright light in your eyes? That is similar to what happens when even the slightest angle is present in foil/metal reflectors. The only way around this is to create a 'wing', which is basically metal bent in angles in a half arc over the bulb. It'll still create hot spots, but they'll be uniformily hot, and more spread out.
Second reason, is that flat white paint is between 85-98% reflective... meaning that very little of your light is being lost. Aluminum foil is between 83-96% reflective, even though that may seem like a small difference... It isn't. Even mirrored glass is less reflective than flat white paint. I know it may seem like foil or metal is more reflective, but that's because it is FOCUSED (hot spots) instead of DIFFUSED. For plant lighting you want diffused lighting... I'm assuming diffused would be the best for fish as well. (especially since us humans do better under diffused too!)
Third reason, is that it's probably the easiest, and cheapest way to line your reflector/hood. Foil is cheap, but you have to hassle with gluing it on, and getting the creases out, etc...
As a side note, when growing indoor plants, the vegetative growth stage (basically, putting out roots and leaves) does best in the 6500k-10000k range (high in blues), and the flowering growth stage (forming flowers/seeds) does better under 4200k light. Not certain that it helps, seeing as how most aquatic plants seldom flower. So keep that in mind while buying your CFLs!
It's funny, I actually have a bunch of huge 100w CFLs... Those are equal to 500 watts of incandescent light. I bet those would just irradiate my plants and fish... lol.
Thanks again for your informative thread!