ok, my uncle recently gave me his whole aquarium setup, a 30 gal. tank, hes had fish in it 4 about a year. i emptyd the water and changed the gravel, ive read tht when u start a new tank ammonia and nitrite appear, the nitrogen cycle, i added 3 Danios to the tank, and the past 3 days ive tested the water and ammonia, nitrite, and nitrate were all 0.. is it possiable tht the tank is already cycled? any info would b great, thanks