Next to nothing,you will need a bottle of water conditioner that should last a couple of months at least and some test kits which again should last months,if you go with the juwel tank then you will need to replace the white pre filter pad when you do water changes but they cost under £3 for a pack of 5.Depending on what fish you keep the food should be quite cheap too "normal " community fish can be fed for a couple of quid a week.As for the added cost on your electricity bill with just one tank i doubt you will even notice a increase.