Welcome to AA!
The first two stages of cycling in FW
, ammonia and nitrite, are toxic to fish, same as SW
, although the bacteria isn't exactly the same. Many members here have had success with fishless cycling using ammonia. I have a 10% ammonia solution that I bought from a chemical supply company. I remember some members saying they got an ammonia solution at a hardware store or grocery store. Just make sure it has no surfectants, purfumes, etc., in it. You can do search here on this site or on the Internet for "fishless cycling".