Where exactly are you shooting from? Inside the tank? Magic gun rays that somehow go through glass without shattering it?
MST3K Mantra. Don't need to think too much about it and enjoy the game. Unless you are really stubborn, just imagine yourself as a Deep Sea Diver or simply just a scuba diver in a LARGE aquarium, like those seen in real life where you can see sharks among other fishes and so present in said large aquarium.
You're a security guard at an official aquarium in a world where aliens hunt fish and all these weird fish exist. The POV is a security camera and the laservice is inside the tank, but just out of view. The tanks you protect in the story mode go on to be different attractions, which is why the story mode loops - you still have a job to do. Your personal tank is either some you got to take home or an attraction you've been given care of for your hard work. The boss lets you have some fish leftover, but it changes every day because the fish get transferred to another aquarium or put in an attraction. The money you make is the budget, and the fish poop currency either as part of an experiment, or it's a metaphor about the fish having very useful poop. That may be why the aliens come after these particular fish as opposed to others - Earth generally has sushi bars. You could also be in space taking care of endangered fish being hunted by an obnoxious amount of predators, or the game is the the ravings of a madman pointing a laser pointer at his aslyum's fish tank.