Let me first welcome you to the Camera Labs forum!
I have just taken a look at your pictures and they really don't seem as bad to me. I like them, really
Here are the things I've notice and my opinions:
- You use the largest aperture possible. It's very good in order to capture as much light as possible but if you'd want a sharper image and a greater depth of field, try using something like f/8. (I think it's also the 18-55's "sweet-spot")
- You use 1600 ISO which is very high. (Try using a lower ISO (say 200) with longer exposures and this will decrease noise and increase your image quality) You would need to mount the camera on a tripod to get the best results and make sure that there is no movement in the reefs.
- Composition is very important for the shots to capture the viewer's interest and as the 2nd photo is still ok, the 1st would benefit from a crop to isolate the reefs and avoid the outside of the tank.
- If you find the color cast to be slightly off, try experimenting with different white-balance setting.
- I'd also encourage you to try using a polarizing filter on your lens as this will remove possible reflections from the tank's glass.
- I also see your main source of light is the one in the tank and it seems quite harsh. You could try and experiment with additional sources that you'd point from outside the tank. This can help selectively brighten some shadow areas.
- Last thing on my mind ... you could also try using manual focus if you ever find that the automatic one has trouble focusing through the glass.
These are my ideas and opinions ... I hope they can help you! Keep us posted on how it goes:)
Best of Luck