Supplementation would probably have to be the ultramarine blue, Hyper Violet, Ultra Violet, blue and royal blue LEDs.
To match the popular ATI blue+ spectral chart, you'd have to use the above LEDs. Their spectral charts can be found at RapidLeds. These will cover from the 400-470 nm range that the radiums do cover but not to the extent the ATI blue+ seems to.
You could probably do fewer Royal Blue Leds because radiums also run with a majority of their wavelength at 440ish - 460ish nm. However, since your wanting to have your tank look somewhat good when the radiums are off, you'll want some Royal Blue in your mix. Once you start approaching the 400 nm wavelength from the 450ish nm wavelength, your eyes start to pick up those less. For example, you won't see much of the Ultra Violet Leds even on max intensity or several of them clustered together. A majority of what you will see will be just the ultramarine blue, royal blue and blues. This combination produces a very BLUE look.