First of all, you don't need ISO 400 speed film. By and large, that's crap film. Actually, you should be using slide film if you want to do anything with your images. If done correctly, there is NO noise from ISO 100 or slower film. Actually, you'll get greater color saturation with a slower speed film. You will need a tripod.
You do need flash if you have any ambient light from the house. You also need flash to compensate for your flourescent light on the tank, unless of course you use a color correcting filter.
Second, if you ever want to know anything about exposing film, you won't use your auto feature. ALL camera meters are programmed to expose film on a gray scale. For example, if your entire image is medium toned (medium gray), then the exposure will be perfect. If you have a yellow tang, for example, that's definitely not medium tone! It's much brighter. Therefore you want to make the image brighter and in order to do that you will have to add more light! You add more light by increasing the amount of light that strikes the film. If you're at 1/60 of a second, you'll need to open it up to 1/45 or whatever the difference is between medium gray and the color you're after. Once you get into this mindset, it's not hard to figure out, even though it may seem complicated now.
Despite the previous post, use flash!! Simply bouncing your flash will do the job. The previous post was wrong if you think you can get a properly exposed image by relying solely on your tank light.