I tried finding a 20 inch pc but the only one I found came with a 50/50 bulb. By the time I order it, and a new bulb and shipping it starts getting to be a bit much for a 10 G. May price the dual incadescent. Bummer is I used to have one and I tossed it, thinking it was useless.
I think I have a pc retro kit that will fit your tank. I'll have to dig through my heap to see. If I remember it's a dual fixture. I'll pm you tonight if I find it.