Goldfish ideally need x10 water filtration. So say a tank is 10 litres, you would need a filter that does at least 100 litres an hour. However, I would aim for one that does 150-200 litres per hour.
You will need to do a lot more water changes than 10% a week. Even if your tank is fully capable of coping with your bioload - which is unlikely, the amount of nitrates that is going to be produced because of the large volume of ammonia processed, will require at least once or twice weekly 50% changes.
Have you got a liquid test kit? If you are set on housing your goldfish in a 2.5 gallon tank, you are going to want to know exactly what your parameters are.