To minimise reflection of the flsh by the glass, shoot at an angle to the glass wall so that the light from the flash is reflected away from your camera. If you shoot directly ie perpendicular to the glass wall, the light of the flash will definitely be reflected back to your camera and affect its exposure measurement.
However, do note that shiny fishes are notoriously difficult to photograph with flash especially if they are very big (or are photgraphed closed up) and are very shiny.
Another consideration is reflections of flash off the back wall or glass bottom of the tank. If these walls are bare, diffused reflections of the light from the flash off these surfaces can cause the photo to look foggy. So to minmise this, ensure that the bottom of the tank is covered with gravel or sand and the fish is photographed against a non-reflective background of plants, driftwood, or other tank decorations, etc.