Can you post a pic of what you are able to get now?
Glass can cost a LOT, so before you start spending let's make sure you've got the basic shooting down. Fish tanks can mess with cameras until you know a few tricks.
To start;
-Shoot straight through the glass, no angles, angles mess with cameras and focus a lot
-Make sure the tank glass is good and clean
-Turn off the flow in the tank
-experiment with turning the lights off in the room around the tank
-Don't try to get too close with the camera itself, it probably needs at least 12" + to focus properly, you can always zoon and crop to get closer.
-Experiment with your depth of field by using the aperature priority setting. Big aperature/small number =s wider depth of field (focus).
I haven't fiddled with that particular lense, but IME you can get pretty good tank pics with just about any "kit" lens. Until you get really good with the camera, it's usually the user not the lense that's the limiting factor (I say that only based on my own personal exp, not taking a jab at you
)