None - So, you want some company and your level of commitment to keeping a living thing - well, alive - is somewhat beyond a houseplant but not quite at the cat or dog level.
You may want to consider fish. You do have a few things to consider first, however. Sure, they don't need to be taken for walks or to the vet for vaccinations, but it's not as simple as filling a glass bowl with water and calling it a home for a little swimmer.
Tips To Consider:
Initial set-up cost is about $200-250 for a 20-gallon tank, filters and air pump, stand, gravel, light source, plants, fish and food, according to the ASPCA.Coldwater fish are recommended for beginners, as they are easiest to take care of.
Fish should have friends. The ASPCA says they should not be kept alone in the tank and as a general rule of thumb, fish owners can have "one inch of fish per gallon of water. That means, for example, you could have 10 one-inch-fish in a 10-gallon aquarium, or 5 four-inch-fish in a 20-gallon. Remember to take into account the maximum size your fish may reach.
"Do not overfeed! Whatever isn't eaten will spoil at the bottom of the tank, hurting the water quality.
Even with a filtration system, several gallons of water should be removed and replaced with clean, "pre-aged" water every week or every other week. The filter should be cleaned monthly.