Dunno if you've ever tried this before or seen it in person, but IME it's going to take a fair amount of 450nm LED power to make a visual difference. (Radiums even at their whitest put out a lot of 450nm light already.) I would ramp up the blue slowly to avoid issues with your coral. Thankfully power can be fairly cheap if you need it...
Looking again at not-supplementing (or at least decreasing the need), what ballast are you running? Radiums really are blue enough - or can be. I swear!
I'll try to make a prediction…if I'm wrong, go pick up some blue LED strips. They will be your best bet!

. BTW, mounting height will determine whether you want lenses or not. U know how high yet?
If you are getting something close to 14000k from a 250w Radium, that should mean you are running a magnetic ballast. And you should be seeing it consume more like 280 watts at the wall.
If so, switching to an electronic ballast should give you the blue you are looking for (as well as better power efficiency) by running the bulb at 250 watts. Craigslist or the For Sale forum should be good sources for cheap used electronic ballasts. Seems like its still around $150 for a new replacement. See how much the LEDs will run and compare I guess.
Just a thought...especially if you don't really need more power, just bluer light.
-Matt
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