Before you start investing a lot of money on new lenses, I'd recommend investing time first in studying the basic concepts of photograpy and post processing (i.e. Photoshop, Lightroom, or whatever software you decide to use). It's a trap many fall into (with no help from the marketing departments at Canon, Nikon, etc.) that upgrading your lens or camera body is going to automatically yield significantly better pictures. Except for some specialized areas of photography, it isn't going to make as much difference as many people think. Other things, as simple as photographing your tank at night when you won't get that reflection of your window in your photography have far greater immediate impact. That's really not a bad photo by the way, especially for someone just starting out.
Bryan Peterson's "Understanding Exposure" is a pretty good intro level book to photography, along with a ton of others. I'm unfortunately unaware of any books that speak directly to aquarium photography, but there may be some out there that touch on it. You can sometimes find online seminars on the subject. I took one a few years ago that was pretty good. . .I can't remember who it was through, unfortunately, but I found it through one of these online forums.
