The only light "penetration" issue involved with fluorescents is related to the reflectors. Light is light, whether it comes from a long tube or a small hot ball. It is the reflector that collects the light from the sides of the bulb that don't face the tank and redirects it into the tank. If it were possible to reduce the diameter of a fluorescent tube to one mm, you could use a linear parabolic reflector to generate a parallel beam of light that would uniformly cover the entire volume of the tank. That, of course isn't possible today.

Water absorbs light, especially light at the red end of the spectrum, but in a 2 foot deep tank that absorption is very small.

Now, if you need to persuade your significant other that you have to buy expensive MH fixtures to get adequate light to penetrate all the way to the substrate, just ignore what I said above.