Please do NOT throw money at a T5i expecting spectacular photos...photography like most other hobby takes time and patience to learn. Although some people may have an eye for photo composition it usually takes years to master, if ever. I have the same camera as you and my photos progressively got better and better. My first tip to you is read the manual, learn everything your camera can do...I guarantee it will have more features then you will ever use. The 18-55 isn't the best lens but its good enough for a beginner, the "macro" label on it is a bit of a misnomer and is on a lot of lenses that aren't true 1:1 macro lenses. Here's a great article that you should read through to learn the basics of photography.
http://www.ximinasphotography.com/lessons/index.html
There are a lot more aquarium specific guides and videos as well online. Section 4 will teach you about white balance which you should figure out first to get your pictures to accurately represent the colors in person. I wouldn't bother with shooting in RAW and post processing until you get the basics down. Here's a shot I took a few weeks ago with the same camera as you:
Good luck!