START-UP COSTS: Starting up a new tank is it’s own cost. I generally estimate cost of a new startup by adding a zero to the tank volume. So for a 55 gal. aquarium, all said and done… tank cost, lid cost, stand cost, gravel, hardscape, plants, lighting, filtration, heater, fertilizers, food, nets, water-changing equipment, water treatment chems, water testing kit, etc.… I estimate a minimum of $550. For a 10 gal. tank, I estimate a minimum of $100.
MAINTENANCE COSTS: Although a lot will depend on how you keep fish, I try to estimate a base of $10 / per tank / per month. The more complicated you make things for yourself, the more costs you’ll run into.