Mount an off camera flash beside the camera at 45 degrees to camera lens. This involves a cheap L bracket and a generic flashgun. If your camera doesnt have a hotshoe you can remotely trigger with a slave and your inbuilt flash (taped over). If you are relying on overhead tank lighting only, make sure the light is between you and the fish so the fish is not dark in shadow.
Although flash can freeze the moment with great sharpness, I personally dont like flash since it can wash colour and blow out highlights. This can usually be corrected with custom white balences but I prefer natural or tank light using a high ISO setting and choose a moment when fish is momentarily still or turning. Aim to have the eyes in focus especially. Some cameras are better than others at high ISO (low light) situations.