The purpose of the reflector is to do exactly what its name implies. Without one, the light that is radiated from the tube would go up the top of the fixture and just be absorbed and wasted. By placing a reflector above the bulb, the light that would have otherwise not been utilized now is bounced back toward the water.
Does it double the light available? Probably not that much since some of the light will bounce back towards the bulb and not make it to the water, but you should see some difference almost immediately. Make sure the reflectors are clean (a lot of people overlook this and the reflectors become less efficient) as part of your regular maintenance.
There are different ones available on the market or, if you are a DIY type, you can get materials like thin mirrored glass or polished stainless steel and make one. Just be sure that ALL of the items you use are able to withstand the heat and moisture. I would recommend the stainless if you go that route and use pure silicone (like GE Type I) to seal it to your fixture.
Why do I love fish? Because every time I stare into the little world I have created for them, every problem I have just floats away....