You have a perfect set up. Now buy yourself a ceramic metal halide lamp (you will have to search the internet for an online supplier). A 400w CERAMIC METAL HALIDE, (not any other kind of metal halide) will put out a spectrum that is just like the sun, and your plants will not only look fantastic, they will grow fantastic because this lamp has an extremely high PAR rating, and plants grow beautifully with it. Mainly pot growers use it, because we insist on only the best weed, to satisfy our stringent requirements. However, all plants grow equally well with CMH. Look up lermer, he is a member here, and is happy to communicate with members to let them in on the benefits of CMH. This lamp has not been promoted very heavily, because there is no huge profit margin in selling it, compared to the profits of some of the other regular MH lamps.

Every CFL bulb I've seen fits in a regular light socket, no special ballast needed.

Mike

NO. HPS fixtures are for HPS bulbs of the correct wattage only. They are NOT for metal halide bulbs, and they are NOT for compact fluorescents. They are also not for HPS bulbs of a different wattage!

Here is a link that might be useful: Grow-Light Primer

nygardener, not always true. I have an adjustable HPS/MH ballast. I can use multiple wattages of either bulb and it will work fine.

"Can a cfl grow bulb fit in a hps fixture, and will it work?"

No.

If not, what alternatives are there (besides buying a new ballast).

I am amazed that you don't already know that you can just screw in a typical CFL bulb into a regular everyday socket, same as for an incandescent bulb. Are you Amish?

"CERAMIC METAL HALIDE, (not any other kind of metal halide) will put out a spectrum that is just like the sun"

The "ceramic" part refers to the aluminum oxide enclosure which has nothing to do with the spectrum. Regular MH have a quartz enclosure. The plasma composition inside the enclosure determines the spectrum.