Yay dslr! Good start.
Love the gray. I had one for over 20 years. Neat bird.
IMHO, your focus isn't all it could be. I suspect you're handholding. With practice you will discover the longest shutterspeed with which you can handhold and still get a sharp picture. 1/50th on the gray even with that short (and hence less heavy) lens isn't fast enough.
+1 for turning off your pumps and using a tripod when shooting tank pics. Once the current and polyps settle down, you'll be able to use much longer shutter speeds (upwards of a couple seconds if necessary), which will allow you to use a lower ISO and / or smaller aperature (higher f number).
And for tank shots always shoot straight through the glass.
For the gray (and other subjects with eyes), try to focus on the eyes. It almost always works better that way, unless you're trying to show a particular feature. The same basic concept appplies with polyps and mouths. Having the mouths in focus generally seems to work better.
Have fun and keep posting pics!
