u/Few-Investigator-702

I have a 40 gallon aquarium with two 1.5 inch comment goldfish. Before anyone says anything, yes I know this is not the ideal size for them, but this is the biggest tank possible per my lease agreement. The fish are ones that my little brother (13m) won at a fair, and when my mom realized how much work they would be she gave them to me. I understand you should not play the goldfish game at the fair, but my middle school brother does not understand the ethics yet. Given the circumstances I want to give them the best life possible, so what is the best filer I can get for them to keep their water clean?