Pre-filter suggestions/cloudy tank

Hey y'all I'm fairly new to fish keeping so take it easy if this is a dumb question. I have a HOB filter and I came home one day to find a fish dead, sucked up onto the side of the piece that hangs down :( So I immediately researched and got a pre-filter to put on it. No more fishies getting sucked up, but now I have a problem of; I don't think my filter is working properly. Ever since I added the pre filter I feel like my water is cloudy and I can't get my nitrates down. Not sure if I just didn't buy the correct pre-filter or if there's a better way, just want to make sure all my fishies are healthy not getting injured too. (It is an established tank, 50 gallons, stocked with mollies and guppies, and some live plants, the filter is the aqaueon HOB one. I have also been adding in some beneficial bacteria during water changes since its happened and it doesn't seem to help much).