Depends upon the size of the bird cage or aquarium. With rats, bigger is better. One problem with bird cages is that rats can kick out their bedding. Frequent cleaning of an aquarium will negate any possible ammonia build up. Also in a large aquarium various play levels can be created to serve as environmental enrichment and exercise. Rats make excellent pets. My daughter's original rat, Henrietta, lived to be 3 + years old - she had to be euthanized because of a mammary tumor. Good luck, enjoy your rat, and protect it from your Jack Russell.