Pretty easy to play :
- click on the big fish ;
- enter your fish name, website if you care and withdraw address ;
- feed your fish a minimum of .1 BTC ;
That's it. You can now click on fishes to attack them. The yellow and green ones are your allies as it were, the red ones you can bite. If you succeed you get half the fish you attacked, if not you lose half your own. The odds of success are a weigh of your relative sizes, but larger fishes can not attack smaller fishes (yea that's right, this is the exact opposite of reality).
Props to him and have fun I say.
PS. I totally owned SatoshiDICE earlier. Ya baby, I took you down from two to nothing. Mwahahaha.