I am unfamiliar with the 3100 but I use a Nikon DSLR. The auto settings usually do a fantastic job. Aquariums are some of the toughest objects you can take photos of. If you still want more control, which it sounds like you might, you will want to manually set your white balance as high as the color temperature can go. On my camera that is 10000K. From the white balance setting inside the menu, you can adjust the temperature even more by bringing up the color matrix and skewing the blue to a warmer color.
In regards to aperture and shutter speed this will not effect the hue/saturation of your photos. It will effect the lightness/darkness.
Your lens will have a sweet spot where it is the sharpest. The sweet spot typically isn't wide open ~1.8 but is somewhere between 4.5~9. Again, it is different per lens and you should be able to google your specific lens to find out the spec. Once you find out that information you can work backwards. Switch your camera to manual. In this example, lets say your lens is sharpest at f9. Set your white balance as high as it will go 10000k and set your aperture to 9. The next step is a balance of ISO and shutter speed. Set your shutter speed to anything faster than 1/60 (anything slower and hand held will produce blurry results) You will want to keep you ISO as low as you can afford (this will produce photos with less noise/grain) while keeping your shutter speed as fast as you can afford. Try not to use your ISO at your highest setting rather a few stops lower. Even though your camera may be capable at taking a photo at a high ISO you sacrifice image quality. Again with the example, lets say your ISO can go up to 1000. Set your ISO to 600 and now adjust your shutter speed to 1/60, 1/100, 1/125 you will need to pay attention to your light meter to see if the frame will be properly exposed. If the frame is underexposed you have options, slower shutter speed which will require a still subject and a tripod, a larger aperture which will increase your depth of field and decrease image sharpness, and finally, a higher ISO which will increase image noise. It's a balance.
I hope that helped a little.