My personal preference is for Canon. I have a T3, but also have a point and shoot. Canon and Nikon (and the new Sony's) are all good and comes down to which features are important to you (from shooting slow motion video to choosing white balance, manual settings etc.) I'm not sure if other people have experienced this, but my photos come out better under T5 lighting as opposed to the LED lighting. I suspect the LED has some effect on the sensor that I'm not aware of- mostly if I'm trying to shoot under the blue lights only.
If you are also trying to get images of your fish, don't skimp out on the SD card- it effects how fast the camera can write to the card. It will increase how many frames/second you can capture.
It's hard to go wrong as long as you find the interface easy to adjust and work with. For me personally, I find it difficult to adjust Nikon's settings, but Canon is easy. My brother prefers Nikon. The quality on both is going to be great.