Planted tank lighting question

So I have a 20 gallon set up at the moment and things are going great! I've managed to keep the water quality good and grow my plants (almost too fast!).

My 20 gallon is using 2 6500K CFL lights in a modified reptile light fixture. However my question is I just got a 38 gallon tank I plan on moving my fish into and the light fixture uses these odd little tube bulbs which I have never seen before. Similar to these here:

http://www.walmart.com/ip/Hagen-Glo-Marina-15-Watt-Clear-Showcase-Bulb-2-Pack/19229873

I don't think a CFL will fit in the socket without cutting it to bits and it says to only use up to a 25W bulb. What are my options? I don't really want to buy another light fixture, but if I had to I could. Any advice is welcome!