[GRR -- just lost a post I spent forever writing... oh well...]
FIRST -- you'll get there! Keep trying! It's GREAT that you have your daughter's tank to practice with, and for some displays it will probably be very similar to what they have, at least in lighting. What you want to be sure to do is turn off all of the other lights, close curtains and doors etc, and turn the tank light on. Any light outside the tank will cause reflections, and aquariums usually keep their ambient lighting low for this reason (at least, they used to).
As has been said, for tank shots you'll definitely want the 50/1.4 and ISO 800 or higher. You didn't mention which Pentax dSLR you have, but I can give you a simplified alternative to egordon99's if we're talking about the k10d. I shoot a lit of band photos which are also often fast-moving objects in a darkened environment (see
Flickr: Photos from amateur6)
If you DON'T have a k10d, STOP reading here! Sorry.
First, the obvious: mount the 50mm, set AF to AF.S, make sure SR is on, set the ISO to 800. Shoot in RAW or RAW + Jpeg if you have big SD cards. I suggest setting the AF point to the center point because I like to remove distractions. I also suggest setting the exposure metering mode to center point, but I may be misguided on that one.
Next, go into the menu settings, all the way over to "C" (Custom Settings). Change the Program Line to "2 Hi Speed".
Now you can shoot in green or P mode!
Try some more shots of your daughter's tank in the darkened room. You should be able to get some decent shots. My tank and most aquarium tanks will have a dark background; if your daughter's is white or bright you may need to change the metering mode to multi-segment. If the shots are still too blurry you can do two things: use a higher ISO (if you have one) or use the +/- EV compensation button to set an EV value of -1 or so (you can get the light back when you process the RAW), and get a faster shutter speed.
Here's a shot of one of my (very dirty) tank residents, an albino cory cat, taken as described above. It's had a little bit of post-processing: Unsharp masking and size reduction, but no noise reduction. Notice that even though he's only 1.5" long, his whiskers are out of focus at f2.4 -- be prepared for a lot of "discard" shots!