Originally Posted by Crepe
I use something closer to 30 or 40 ISO. Then again my lights are T5HO and the last picture in my first post is what things look like in my tank at that ISO. A lower ISO in my experience lets you take nicer shots of fish because there's less chance of graininess due to a small aperture, additionally there's less glare and less blurring when the fish move. But I'm not photographer, so that's the extent of my knowledge.
You are correct that it's better to keep your ISO low and try to balance your aperture and your shutter speed first. Your aperture controls how much light is let in and your shutter for how long. Generally, you want a higher shutter speed if you can because they will result in sharper images. You want a lower/wider aperture if you're photographing people / pets, etc. because then your depth of field is lower, meaning that you're likely only really focusing sharply on one feature (with people, you want to try to set that on their eyes, for example). However, with large groups and/or landscape, you want to have a higher/smaller aperture because that will allow you to have multiple sharp focal points. (And of course, opening your aperture is the easiest, and should be the first thing you do when you try to increase exposure, in my opinion. But watch for the clarity of your image, because after lighting, sharpness is the most important thing.)
However, if your exposure isn't right even when your shutter speed and aperture is where you know is optimal for your shot, you can increase the the ISO, which will come at a cost of grain, however, where grain becomes visible depends on your camera (for me it's around 800-900 before you can even tell). ISO doesn't really affect clarity of the photo all that much unless you're starting to get grainy from going too high.
Definitely, the better lighting you have, the easier this whole process will be.
And sorry for hijacking, redfisher! But as OhNeil said, having your camera set up properly is important, so maybe this will help someone else get a killer photo to add to this thread, too.