Hi. It's hard to say whether you need a second filter not knowing the size of your current tank and filter, as well as the size, type, and number of fish you have. In general, one filter of the correct size is all that you need. People often over filter their tanks which is fine, and even beneficial. You just don't want to have too much current. So as long as your filter can pump around 5 times the volume of water in your tank an hour, you are probably ok.