I build grow boxes for people locally, and after much research into reflectors, only two things are worth mentioning-
both are by far the two best things for reflecting light to plants. (underwater could be different, but I doubt it)
See, you don't want reflective, as much as you want refractive. Reflective appears brughter to the human eye, but refractive will spread the light out, ensuring even distribution into the tank.
The two best things are:
flat white paint (reflects about 70-80% of the light, while ensuring good distribution
Mylar sheeting (only slightly better than white paint.
Anything else will produce "hot spots" where the lighting isn't as even.
Of course, most of this might not even apply in an aquarium, since the water surface could refract the light. *shrug*
The trick is in not trusting your eyes, using the reflector to shine on a wall in a dark room works a little bit better, but a thermometer with the end painted black would probably be the best. (black would absorb the light, convert to heat, and give you a number to use to compare)
this is probably way too much work for an aquarium though.