Best way to do it is stand in one spot, rotate from left to right (or right to left) making sure that each successive photo has a fair bit of overlap with the previous. Some cameras have a stitch assist mode to help you line up the features. If yours doesn't have that then make sure that maybe around 25%-33% (1/4 to 1/3) of the photos overlap. Also you don't want too wide of an angle with the zoom (ie., zoom in a little) so that it doesn't distort out towards the edges.
Next use a photo stitcher software. If you don't have one, Microsoft has one you can download for free called "ICE".
http://research.microsoft.com/en-us/...roups/ivm/ICE/
I'm using this one myself and like it. It has a couple different stitching algorithms in case one doesn't work (sometimes the angling can be hard to compensate for).
I wasn't able to stitch the two photos you posted, possibly they are too far apart. My suggestion is try again but use at least 3, maybe even 4 photos for the same overall image and it will probably work a lot better.
With fish tanks stitched or composite panoramics always are interesting since fish move around and end up showing up in multiple spots in the tank.
Or end up being half-ghost. So try a couple different times and have some fun with it.