If you are going DSLR, couple of questions:
1. Budget
2. Intended use other than aqarium photography.
I have purchased all my gear used except one lens. I have never shot the T4i, but it would be a good choice looking at the specs. The one part I wouldn't like specific to aquarium shooting is the WB'ing options - there is no temperture specific setting - I set my 7D to 10,000k which gets me pretty close, then do the rest in post. You could custom WB though, will just take some trial and error to get it close, then you can further adjust in post.
As for lenses, I agree with Darryl. There is no one lens to "do it all". 50 to 60 is a good lens, and will get you some great shots, and will be a better all around portrait lens in small rooms, but for closer shots a longer focal length will be better. Bill got a 70, which is a good intermediate length IMO, and I have the Tamron 90, which I use for all of my aquarium shooting, other than full tank shots (if this is what you are after, then yes, you are going to have to stand way back, but you are going to have another lens other than the 90 mm for this, I'd suggest something in the 30 to 50 range - I have a 30 mm Sigma 1.4). Anyhow, I love the 90 for outside close-ups, coral shots, macros, and fish. Few samples: