for fish, definitely shoot in manual. and there's no better way to understand photography than to force yourself to shoot in manual.
dont use those little icon shooting modes, theyre worthless. i started in S and A modes, now everything i shoot is in manual mode, even everyday pics. afterall, that's the whole point of a DSLR
in manual you need to tweak your shutter speed, aperture, and ISO to get the right exposure. given that you dont have a speedlight right now, your limited to low shutter speeds, high ISO, and wide apertures, which doesnt always result in the best photo.
and you also need to adjust your white balance, but this is easily fixed in post processcing