If you adjust the ISO down you'll need a slower shutter speed to compensate. Using Av (aperture priority) is a good way to go for this stuff. The smaller the aperture (higher number) the more depth of field you'll have; i.e. how much of the scene is in focus.
Sometimes you want a lot, sometimes you want the background to just blur out.
I think you have room for color correction in the first pic. What strikes me immediately is that the image looks rather yellow overall. If you correct the white balance using the lighting of your aquarium against a white surface in the tank, you should end up with pictures that have much more color definition. If that is a green slimer than it should look really green. I would like to see you retake that image and repost it.
Also, if you drop the ISO you will either have to increase the available light by using a flash or increase the exposure time by using a tripod.
Hi there and thanks for your input. The acro in that picture is really that yellow but the polyps have more of a green tint to them than in that picture. The green is subtle but it's still there. Using the advice given on another thread, I used a gray card and positioned it in such a way that the tank lighting was hitting it then I snapped a pic and used it to set the white balance manually. When I used the auto white balance on my camera, this is how the picture came out:
The color of the polyps in that picture is more accurate but not the color of the flesh of the coral which is bright yellow. I plan on practicing with lower ISO settings later tonight if I have time and will post new pics. Thanks again!