I keep the flash on and hold the camera at an angle so the flash doesn't get into the picture. The flash usually helps with skittish fish so that when they keep moving they are not just a blur.
Yeah, just make sure that the glas is clean (inside and out).
Make sure to aim the camera slightly downward to prevent the flash from reflecting into the lense.
Make sure there arent any lights around the tank that will creat a reflection on the glass
But DO have your tank lights on.
If you have a manual focus, that helps the shot look a little clearer.
In your picture the auto focus is focused on the stump and plants and not the fish. Let your finger back of the button and re-focus until the fish is the one that's clear.
i have the same problem as well..sometimes i just cover the flash with my finger it takes away some of the flash but still lieaves enough for it to be affective...kinda hard to explain lol