too much light?

[Delphinus]

One technique that you can do is keep your photoperiod at 12 hours, but the light is never on for more than, say, 1/2 an hour or so. So half-hour on/half-hour off/repeat 11 times; kind of thing. Then after a week it'd be like 45 minutes on/15 minutes off/repeat ... and so on ... gradually ramping up to a full 12 hours on (or whatever photoperiod you're going for).

Another thing is to raise the lights temporarily and lower them gradually every few days. Might not always be practical though, who knows. But it works pretty good!

I've also heard about using eggcrate on the top of the tank, or even stacking eggcrate. Creates a wee bit of shade and you pull away a layer when it's time to ramp up. Never tried this method myself though (I have tried the other two ideas up above and they worked out well).