a small internal filter could help to keep the tank cleaner but as other ppl have said before me goldfish need big tanks (its a common misconception they are fine in a small bowl or tank but this is plain cruelty and petshops should be fined for selling fish bowls) i would look seriously into rehoming (or buying a bigger tank or putting in a pond if finances permit) the goldfish and setting up a tropical tank with small fish as there is a wider variety of more colourful fishes taht would be at home in a small tank(a lot of these fish are easier on the wallet too) it is a another common misconception that tropicals are harder to keep than goldfish, this is simply not true(majority of common tropical fish aneway) the main difference is a heater which is not that expensive, the other being (as said before) goldfish are messy eaters and require effiecent filtration and more maintenance(cleaning etc). im not trying to put u off fishkeeping and im happy u have stumbled across this site as u will find out most of us here are happy to answer your questions and welcome you to the hobby , 1 tip i can give u is to read the threads marked 'sticky" at the top of every topic as some of these contain more indepth looks at things like setting up and cycling tanks also u shopuld watch out for a diesiese most of us have on here , its called MTS or multiple tank syndrome u will find 1 tank wont be enuff soon