Can One & Only be used for fishless cycling?

Yes, but there is no real reason to do fishless cycling with One & Only. However, if you choose to go that route you need to add ammonia to the water after dosing the One & Only because the bacteria need some source of ammonia to start growing a bigger population. Dose ammonia to an initial concentration of 2 to 4 ppm which should disappear quickly, with nitrate increasing a little then disappearing. Dose one or two more times with ammonia and after your test kits show no ammonia and nitrite, it is safe to start adding fish.