As I understand it, the higher the number the more UV is in the light. A spectrum above 460 is not tolerable for our corals. Which is why we can't use "Black Lights"...they have too much UV.

The 420nm will probably be perfect for you, I think the 450nm will be a darker blue because of the red uv. I use 450 for actinic suplemant and it works perfect with the 14k MH bulbs I have but I think it is too dark of a blue when the MH's are off.

My understanding of the spectrums could be a bit off, but i think it's pretty close.