How much did you pay for the 55-200? I just got a deal on amazon for a 55-200 for $94. I also got a d3100 which should be arriving soon. Maybe you could get it from amazon if it's not too late.
Your new lens is a telephoto. think of binoculars. It will be good for wildlife. If tank photos are your goal, you want a macro lens. Think magnifying glass.
Gorgeous corals by the way!
I got my camera in! Here is a quick shot, unedited taken with a Nikon 85mm f/3.5 G. I didn't use a tripod so its not the best, but it looks decent. I'm also throwing in a couple random pics. I will post up some stuff up and link it soon.