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.