Use your keyboard "up" and "down" buttons to control the fish.
The black boxes help you to prevent collision with the right wall, but if you stay too much in front of a box, you will be hit by the left wall; also watch out for the city, don't get too down, as you may lose health as well as you lose when you hit the upper wall. So the main idea is to resist as much as possible.
When your health is too low, you may increase it by hitting health pack which automatically gives you 100 hp.
Also, watch out for the green boxes because you lose 10 hp after each time you hit them.
Notice that your score increases slower when you are on a cube than when you are flying free. After you finish, and your fish explodes, hit send, if you want to send your score. If you feel your score is not enough, click reset