Focal Length 105
FNumber 4.5
ISO Speed Ratings 500
Because you were all the way at 105, your aperture is only going to open up to F/4.5. That's pretty slow for stop action low light, so to compensate your ISO automatically bumped up to 500.
500 is pretty grainy, as you can see in your second picture.
A few things you could do:
1. Way more light somehow
2. Get a faster lens (f1.8 would do well to "stop action")
3. Zoom out all the way so you can utilize the bigger aperture for a faster shutterspeed
For stop action fish shots, I usually find my 50mm F/1.8D to work great under full on metal halide lighting. 4.5 isn't going to cut it for the dim lighting our tanks have... unless you have a 400-1000 watter going on =)
keep going at it! looks great. btw are you coming on sunday to roam around the city with us?