AFAIK, they're not compatible - every type of fluorescent (and there are SO many!) has its own way of fitting into the fixture. There are lots of retrofit kits available online, if you want to look back about a week I think there was a post in the Products & DIY section about this. You could also start a new thread there and someone can probably tell you where to get them.
Hope you don't mind an opion, I'd forget VHO and go with Power Compacts - they were recommended to me when I went planted and they're great. I'm using a 36" fixture on a 45g tank, it's 110W and very bright (PCs put out more light per watt than regular fluorescents), the plants are doing well even though it's a deep tank.
