Getting a tank and buying a couple of fish is easy enough, and fish can be incredibly low-maintenance if you're looking for calming animal companions.
The biggest cost for owning fish is purchasing the equipment that the fish will live in.
Up-front costs include $200 for a tank and equipment.
Food costs very little, in the ballpark of $20 a year, and you may have some miscellaneous costs of $15 for other tank accessories.
Your annual total is about $235.
(Information sourced from ASCPA and Petfinder)