elhazar:

Fish are not cuddly, they do not have soft fur nor do they have cute, sad or otherwise expressive faces. Yet they are living, intelligent and deserve respect as well as proper care: Learn about the Nitrogen Cycle and cycling!

The Nitrogen Cycle

Fish and other aquatic animals as well as rotting organic matter produce deadly ammonia: If allowed to accumulate, they will literally shit themselves to death, quickly. However, beneficial bacteria transform that toxic ammonia into toxic nitrite into mostly harmless nitrate. In aquariums, the accumulated nitrate is removed with water changes. So before you can get any stock, you need to cycle your tank, to grow your colony of beneficial bacteria. However, they do not appear from alone, but you rather need to feed an unstocked and filtered tank ammonia, for example from decaying fishfood, to grow them. After about a month of doing so, you will need to confirm your bacteria colony on your filter media can transform all ammonia without leftovers into nitrate, only then your tank is safe for fish!

Also, goldfish, even the cheap ones you see as feeders and the carnival-prize ones, get to almost a foot long and need a massive tank. Fancy goldfish need at least a 30 gallon tank. Bettas should never be kept in anything under 5 gallons and need a heater. 

Do your research before you get any animal, even a cheap one.