Light Diffuser

It's hard to think of anything you could do to a standard fluorescent tube to diffuse the lighting. Standard tubes are a pretty diffuse source of lighting as it is, they produce diffuse light by design. What you are perceiving as a lack of diffuseness would be, I imagine, more a factor of how much light there is. Even twin T8 lights, the cheap basic sort, produce a lot of light considering the volume of the tank. If you have real plants, you shouldn't mess around with this; But if you don't, you could try removing one of the tubes. The problem is that some fittings will refuse to work with only one tube.

It might be worth giving some natural floating plant a try if you're concerned about the harshness of your lights - even if they gather at one side of your tank this will at least provide partial cover and give your fish the choice as to which lighting conditions they prefer to be in. I have floating plant pushed to one side by the filter flow and find that my cardinal tetras particularly like to gather underneath it.

hi, any idea what they are called?

Vallisneria of a number of different species will do that, too.

These are my sprawling ludwigia (with a few fuzzy fish in the foreground). At least I think it's ludwigia......