It sounds like a very exciting book! Is the time period modern day? Do they have computers and cell phones and such? It will be interesting to see what happens to the fish and what the problem will be! Mark sounds like his a pretty mean guy to poison his friend's goldfish! Is Tom a grown-up or a kid? Does he live with Mark? Make sure that when you reread your post, you check for capital letters. Nice job! Adding a few more details will help me understand more about your characters.