I have a 5-gallon tank with a built in filter and it is AWFUL. I have to do water changes every day because the filter can't do its job! I currently am using Aqueon® QuietFlow Internal Power Filter as a replacement (10-gallon size, it was the smallest one I could find) but I'm not quite happy with it. I'm looking for a good filter that hangs outside the tank, not the ones that take up space in the tank. I also would like if the filter could be compact and hidden, but I am willing to sacrifice the look of a filter for one that can work. Any recommendations?