My preferred tropical fish shop is Maidenhead Aquatics at Hickstead. Very helpful, friendly and knowledgeable
Second that.
Going North, turn off the A23 at Ricebridge then follow the signs to rejoin the A23 to London and on the left of that road (which joins the A23 after about mile) is the Maidenhead Aquatics. Massive selection of everything you could need. Everyone I know who has fish goes there (unless they are looking for the battered variety)