You put a mammel in a cage, change the bedding now and then, and it is going to be ok, you can debate that it needs a larger cage for exersice, and most of the time they probably do, the assumption is that you are going to take it out of the cage and let it exercise.
Keeping fish means
YOU are creating an entire world.
YOU- are creating a biological cycle in the filter and maintain that cycle to create safe cycled water with bacteria colonies that developed to destroy ammonia and nitrites, and turn them into safer nitrates.
YOU - have to carefully plan for the amount and fully grown size of the fish, The temperment of the fish, the specific needs of the species, the amount and type of fish food, the size and type of filter.
1. The larger the tank the easier this is.
2. Some fish need more space then others, some fish will get get stressed if they are overcrowded, or if they do not have room to swim/exersice., Stressed fish can be nasty and aggressive. Fin nippy, tail chasing , or simply lethargic, all stressed fish are open to disease like ich.