fish botlane is a pretty intensive lane as you are a melee champ with a very limited mana pool. Use fizz's mana incredibly wisely not going for too many E's during laning. use it mostly for defensive reasons - such as better positioning or backing off. You can use it aggressively however by using it to gap close to prepare to use your q and dive them like that. Fizz is super good at getting cs with you W

pro gamer tip: whenever trading you can press w and it will apply its on hit ability on your q.

another tip is whenever goign for a little bit longer trade, press q, aa, w, then e (dont recast) for a slow, then rush them down if they are low.