Well I'm assuming you're using a point-and-shoot camera or cell-phone camera. With these sometimes it's best to just plan to take a ton of pictures and find the right ones that are focused. Could take 100-200 pictures and only get one decent one. Also some people will take a video and stop it and screen shot it. This usually is not as good quality of the picture alone. I hope that helps.
Try recording a video, and then pause it when the target fish is in focus, screen shot, save the photo, done.
Option 2 that works as well is taking a small chunk of rock, and a chunk of frozen food, rubber band it to the rock, and let it sink. Make sure to drop the food rock in a good area for u to focus in , proper lighting, lower flow, etc... once the fish start hovering and pecking, take as many photos as u can and sift through them later. Hopefully you will have a few good ones to choose from.
Only way I have ever even come close to taking a picture of a fish is with my husbands camera and a tripod on a fast speed setting. He sets the camera up, I just click randomly. I got a couple today, but they are not that good. Good luck!