Floating plants, how much light reduction?

[Tomfish]

I'm having few issues running 2 t5s over my 4ft tank. The lights are fixed so i need another way to reduce the light. I'm thinking filling the surface with maybe amazon frogbit. So how much would a full covering reduce the light? I'm trying to grow a hc carpet, so a little worried that a too dramatic cut in light could cause it problems. Any thought would be greatly appreciated. Cheers

 

[jojouk]

Interested to know as we'll

 

[dean]

If the frogbit covers the entire surface I'd say it would cut the light by atleast half probably more, it's simple maths really, cover half the surface= half the amount of light available as only half the surface area is available,
Good thing about floating plants is that they are a good quick indicator of your lighting and fert regime