UGF's work, and after you clean out the tank, if it was adequate for SW it will be adequate for FW. However, you will find that many, many FW fishkeepers prefer HOB or canister filters instead of UGF. Remember, the SW nitrifying bacteria are different than the FW nitrifying bacteria, so you can't just swap out the water and add fish. The mulm in the SW UGF should be removed, gravel cleaned, FW added and start from scratch.