UPDATE!

This was one of my favorite simple apps but the developer(s) havent updated this since Ive had it on the 3Gs so it looks terrible and has functionality issues (obviously)... Its a shame this was abandoned...

However there is another app called iFish Pond that has very nice realistic graphics and some fun options, the only downside is that the fish wont nibble your finger lol...

Sephiroth726 about Koi Pond, v2.5