Very nice pics, especially for your first attempts with a macro lens. Here's a few tips for macro shots of fish:
Your best bet is to shoot in manual mode and bump up your ISO slightly (try to keep it less than 400, if possible to prevent grainy images). You'll have to adjust your shutter speed accordingly, depending on how mobile your subject is. If you have a flash diffuser or bounce your flash off of another object, you'll be able to increase your f/stop and get more of the fish in focus. However, if you go too high on your f/stop, you'll lose a nice bokeh effect and it will appear that you were trying to get the background in focus (unsuccessfully, I might add). Shooting straight on with a flash will result in a big flare somewhere on the image and you probably won't be able to crop it out of the final image. The only way to use the stock flash would be to shoot at an angle and then you'll have difficulty focusing properly. I always found it easiest to use manual focus and chase the fish doing my best to keep the lens perpendicular to the glass. Autofocus is great for some fish, but many of them move so quickly that your AF will never really lock in unless you get lucky. As far a focal point, try to get the eye to be the most in focus and let focal length adjustments do the work beyond that. When looking at a photo, most people will immediately go to a fish's eyes to guage focus and it will result in a more visually appealling image. When it comes time to post-process, keep the rule of thirds in mind for cropping to get good composition and before you know it, you'll have magazine quality shots! Keep em' coming Franky!
