[Canadian Man]

It's not necessarily the actinic lighting itself that's important. It's the specific spectrum of "blue" light which corals need.

So if you use let's say a halide "iwasaki" bulb over your 50g cube tank as your only source of light. You corals would do fine. That bulb provides a full spectrum of light which contains enough "blue" spectrum for corals.

If you added some actinic supplementation to your Iwasaki bulb it would make the overall appearance of the tank seem more white and at the same time increase the "Blue" spectrum of light. It's not necessary but it may help with coral color and Coraline growth etc.etc.

So no it's not " always " necessary but it depends on the situation.

cheers