Yesterday I installed a Tetra Whisper Power Filter 20 in my 10 gal which contains 3 Peppered Cory Cats and 5 Neon Tetra. The water was murky when I installed the filter. The next day the water was still murky. After leaving it on overnight at max power I don't think my water should still be murky. All the parts are working, water is being filtered through.
Maybe your tank is just murky lol. Could be anything. Your filter, your water, your decorations, your gravel, your tank windows, your eyes, lol. Try partial water change and check all the aforementioned. Process of elimination :)