A tripod looks to solve your problem. as for image size, its hard to tell on a monitor. As a rule,Whatever your focal length is, your shutter speed should double it, for handheld, but if you don't have enough light your picture will be underexposed, or look really dark. the only problem with shooting with a slow shutter speed, your fish may blur if they are moving, your color will be really good though. Or you can fix it in photoshop if your image is in focus.