Nice pics, especially the first pic. My recommendations as far as DSLR usage:
-If you can help it, never use any sort of automatic shooting mode (except maybe A and Tv) until you have learned to use full manual mode
-the first pieces of equipment you should buy for aquarium photography should be a good speedlight, flash triggers (I use these:
http://www.amazon.com/CowboyStudio-...IXZW/ref=sr_1_1?ie=UTF8&qid=1324946936&sr=8-1), and either a dececent sized softbox for overhead lighting, or the materials to make a DIY one (my ghetto setup is made out of a styrofoam box with a rectangle cut out of the bottom and a blank sheet of printer paper as a diffuser). All of this stuff will allow you to utilize an overhead flash, which will make the process capturing quality photos much, much easier.
- read up on the usage of manual mode; it can be pretty confusing at first (at least it was for me), but there is a lot of helpful info out there in photography magazines, books, and websites.