First of all, you don't cycle water (or filters or tanks for that matter) you cycle filter media.
Cycling with fish always carries the risk of exposing the fish to levels of ammonia and nitrite that can kill or cause long term health problems.
Fishless cycling allows you to grow the good bacteria safely, as there are no fish present to be affected by the toxins that will be in the water untikl the bacterial colony is large and robust enough to eat them.