Best settings for low light area?

I'm doing a project for school on the Baltimore aquarium so I need pictures of it. I'm going down there this weekend and I was researching best settings so the pictures will come out well even though the lighting is super dark in there. I have a Nikon D5100 with the lens it came with (18-55 f/3.5)
From what I found out, it's Better so have f/2 or lower.
I'm new to this camera so I'm not really sure what my settings should be at. I already know I'm gonna have to keep the ISO between 400-800 so it won't get too gritty. But i don't know what to put the other settings at.

Well you're not going to get f2 with that lens. At 18mm the best you'll get is f3.5, if you zoom in, then at 55m the best you'll get is f5.6.

You are going to have to up the ISO to get a fast enough shutter speed to-
(a) handhold the camera
(b) make sure that any fish movements aren't blurred

I'm guessing you'll need at least 1/60 sec, possibly more, so you may have to have the ISO higher & deal with the noise in post processing. It's easier to deal with noise than it is to deal with blur.

If you can beg/borrow or buy a faster lens it will help (50mm f1.8 lens is less than £100, but you would have to manually focus it on your camera).

You may find that even if it's dark in the area, there will be illumination in the tanks - in order to prevent glass reflections, it's best to press the lens flat against the tank glass to prevent those & it will also meter in the tank better.

You're right about needing a high ISO and a fast f1.8 lens would definitely help. If a new lens is just out of the question you can use a program like Noise Ninja to help deal with the noise from a high ISO. Another idea that will help is to use spot metering. Keep the lens against the side of the aquarium tank and by using spot metering you'll only meter the fish.

Watch: http://www.nikondigitutor.com/eng/d5100/index.html

I would use the asa/iso setting of at least 800 and use at the slowest shutter speed of 1/30 second with lens wide open or up to one stop closed with a proper meter reading/camera setting.

If i can't get that 1/30 second lowest hand holdable shutter speed to use with lens set at wide open or a stop closed down, I would up the asa/iso settings to 1200, 1600, 2400 and 3600 until I could use that slowest hand holdable shutter speed. And I would fill the frame with the full image I would use after the picture taking session. That is, compose the image right in the viewfinder and use the full image view as the final picture.

NOT CHOOSE A SMALL PORTION OF A PICTURE IMAGE LATER, AFTER THE PICTURE TAKING SESSION!
Why? Because you are taking pictures… Yes. But very poorly if you do this way. And the image quality will degrade quickly. And you are degrading the quality images a Nikon camera is capable of making! Poor! Poor! Poor technique!

So. Compose the final image with camera held horizontally, or held on it's side, long side up and down to compose your images. And zoom in or out slightly to fill the frame with the final image you can imagine seeing hanging on the wall or on a computer screen… See that complete image filling the camera viewfinder.

And use the features built into the camera to produce a quality image.

And one last UM… The zoom lens f 3.5 may not be suitable for such low light work. You may find out the hard way. And hang your head in shame forever.
Best you go and take photos to first of all find out if you can get usable images before doing the work for real.
If you find the single zoom lens is not suitable for the low light work, bow out gracefully and tell whoever you can't take the pictures due to your equipment limitations.

Or buy prime lenses with apertures of say 1.4. Or at least a f 2.0. A set of lenses to use would be a good wide angle lens, normal length lens and a short telephoto lens.

Low light work is very specialized photography. And you get the best images with prime lenses with large apertures. Use high speed films or set digital asa/iso at the highest to make usable images with no camera shake at the very least.

Don't be discouraged if you can't make usable photos with the lens you have in such a low lighting area.It, the lens and camera, is perfectly good for other well light use/photography.