Scarin-
I would venture to guess that the "graininess" is from using Picassa to process the pictures. An ISO of 200 is not going to add much grain to the picture in the same sense that a "film" camera does. And, if it did, it would not be noticable. With that being said, you usually do want the lowest ISO setting as possible.
Once you get your Photoshop software, you will have a much better tool for processing pictures. It is immensely more sofisticated than Picassa. As a matter of fact, it is capable of so much that it is overwhelming.
It sounds like you have the basic equipment down. The one thing I would recommend, is the "remote shutter". I believe there is a tethered version that will plug into your camera, and a wireless version as well...although I am not sure, because I also shoot Cannon.
Turning off the current in your tank will go a long ways torwards the capture of those "tack sharp" pictures. Also, try to always be straight on with your subject. Angles always show up as distortions. Shooting at night with all competing light sources off will also garner better results...and I am sure you have figured out...never use a flash.
The other thing I notice between my computers, is the difference in the color display between monitors. On my laptop, the images alsways look more washed out...on my desktop display, they always look oversaturated. There are specific software calibration programs to "adjust/correct" the colors of your monitor. I haven't done this yet, as I am waiting to upgrade my desktop and monitor...
...this is another tangent of the hobby that can be/is more costly than reefkeeping itself. The biggest ingredient to a great photographer, is "shoot, shoot, and shoot. It is very common to take 50 shots...and be happy when 1 or 2 of them are usable. As you get more familiar with your camera, and what settings are appropriate for the effect you want, relative to the conditions of the subject, the ratio of trash photos to usable photos, gets better.
Hope this helps some...
Scott
