You have 3 goldfish how often should you clean the 1.5 gallon tank?

Once every fortnight, depending on how many fish in your tank. However, the filter should be cleaned only twice a year. This is because the 'good bacteria' in the filter would be washed out when cleaning.

Do not use any chemicals in your tank when cleaning. Except Water conditioners or Water agers. They are important and should be used every time you do a water change. Also. When doing a water change, Only take out a quarter of the tanks water
every week