To change the background press space or the number 9. To move the clown fish up press the up arrow. To move it down press the down arrow. To move it left press the left arrow and to go right the right arrow. For the pufferfish press Q to go right, W to go left. To go up press A and press S to go down.
I hope you have fun messing around with the aquarium and by the way this is my first project so I'm sorry if you don't like the quality.