All these experts, and nobody commented on the fact that the OP said that he doesn't use a tripod. Even the steadiest of hands cause camera shake when shooting macro - the best thing to do is get the dust off that tripod, and even use a remote trigger release or the 2sec timer. getting your hands off the camera will help clear the image up. also, make sure to shoot as perpendicular as possible through the tank to minimize distortion from the tank glass. Just my two pennies.