Nice pics for a first time effort, better than most I have seen.
1: Try not to use flash
2: Practice
3: Get a nice tripod
4: Practice
5: Use Av mode with Tripod and Iso 400 to begin, lower it as you get more confident, use Av (aperture) of f8 and go from there.
6: Practice, hehe
7: Turn off pumps and keep lens 100% in line with tank, any angles between lens and glass will distort like a mofo
8: Oh did I mention practice..
You are on the right track, keep it up!