What is the best for a nano tank, 400nm or 460nm?

Just curious.

In regards to what?

We need more info.
I believe the 400nm is going to be more of a darker purple color and the 460nm should be more of a blue color.
I personally haven't seen a 400nm before but maybe I have never actually been looking for one. Lowest I have seen is 420nm.
Coral should pop pretty nicely under a 420nm. I dunno about a 400nm.

Here is a little article explaining basics of actinics on corals, basically to the human eye the 420nm will look more purple, but corals don't use that wavelength.

Actinic help


Here is a good example of colors:


From left to right: 420nm actinic, 10k daylight, and 460nm actinic

Thanks to OceanNano for supplying the picture.
You can click his name to bring up pictures of his nano.

But, is this wavelenght harmful for fish and corals? Or it's just useless?
What if use a 10.000k lamp, an Actinic 460nm bulb, and these 400nm lamps i found here, is there a problem?

Well, there isn't 400nm bulbs. Actinics stop at 420nm. They aren't harmful for fish or corals.

460nm is a blue bulb and is primarily used to help raise the color temperature in the tank, and adds a little pop to the colors. 420nm actinics are a violet color and really help pop the colors in corals. Neither bulb will add any appreciable amount of PAR to help with coral growth.

460nm PCs wont add par?
confused now
why not

It's very little typically, and it changes a lot from manufacturer to manufacturer. The phosphors used in most "blue" bulbs radiate very little light. The relative intensity is quite low. Some of the newer ones, like the Current USA actincs and the UVL 454, do put out reasonable PAR, but Coralife ones that I have tested were pathetic. It's all down to the phosphors used. It's a different story with T5s though.