Wow! Stunning! They really suck.
Okay, lets see if I have anything constructive.
First, your DOF is too shallow. The foreground and background is blurry in most of them and just a small portion of the center is in focus. Your going to need to stop down some. Try f8-f11. Aperature priority. You need a tripod if the shutter is slower than 1/125 of a second. Boost the ISO to 800, see if that helps.
Your Zoos and Hammer look like they were moving. Shut off your pumps and let things settle. It will help with motion blur.
Use a tripod and timed shutter release.
When funds allow, get an off camera flash and a shoe cord. The Canon 220EX is a nice little flash and only costs $128. With the shoe cord you can hold the flash above the tank. Makes some nice effects and will solve alot of lighting problems.
But for now, work on getting sharper picture. Its all technique. It took me months to get decent.
Tip: Learn the functions of ISO, f-stops, and shutter speed. Then shoot on the manual mode only for a month. It will teach you all about your camera. It will also train you to always check settings in the viewfinder before you snap.
Mike