Do you want something off the shelf or a DIY solution? I have no idea what is available off the shelf for the 1.5 foot sized aquariums. In fact, it was quite difficult to get the 2 foot 36 W PL-L fitting that I am using since not many LFS carry it. I suggest starting with a 36 W PL-L and see how it goes with the riccia although due to the length of the tube (which is already 18 inches long) I am not sure whether there will be any fittings that will fit your aquarium. Of course, you can get the longer fitting and have the ends sticking out. East Aqua in Ikano also has some fittings which look like a spotlight and is powered by bulbs that look like the normal energy saving type although I am not sure how effective they are since they don't have proper reflectors.

You will have greater flexibility with the DIY approach and there are several forum members who will be of help in this regard.

Happy New Year!