The watts per gallon rules are not very effective ways to judge if there will be enough light or not. If you have 48 watts of PC light you should be OK with soft corals. Zoanthids could possibly lose color and probably wouldn't grow very fast but will most likely not die off due to lighting.
If you use 48 watts of T5 lighting and the proper reflectors you will get more light directed to your corals and, in turn, have better coloration and growth and be sure to keep all softies and most LPS and maybe even some easy SPS.
The tubes of a Power Compact are curved back next to each other and the light that comes out between them is pretty much just lost in the fixture bouncing around. The reason T5 lighting is so much more efficient is because the light that goes up is bounced perfectly back toward your corals, and the light that goes out in any direction but down is also bounced toward your corals. There isn't any obstruction to block the light. The single bulb reflectors are computer-designed to have the perfect shape for a T5 bulb.
what is the depth of your 20g tank? That is the most important dimension when dealing with light because it is filtered out by the water as it passes through so the light at the bottom of a 24" tall tank will be much less intense than the same light at the surface of the water.
If you end up with that PC light and have trouble with zoanthids you could try moving them up very close to the surface and keep the other corals lower if they are fine there.
There is a good deal on a power compact fixture in one of these for sale threads if you can use a 36" light. I think it is 2x65 for $50.
With that said I think even the cheapest of T5 fixtures will be more efficient at lighting your corals. Just because your fixture is CONSUMING 130 watts that doesn't mean your tank will RECIEVE 130 watts. I have seen guys replace 440 watts of VHO lighting with 216watts of T5 lights and be surprised at how much brighter their tank is.
Just remember, in this hobby reading up on things before purchasing is your best move and could save hundreds or thousands of dollars down the road.
Hope that helps,
-- Kevin
And thanks for the welcome.
