I was given around a dozen platies roughly 3 weeks ago to help cycle my 55 gallon tank, throughout those weeks I tested my water every other day or so and I cleaned the gravel and did small water changes to keep my nitrites low enough to not kill my fish, today my nitrites dropped pretty damn low and my ammonia was around the same ( I kept ammonia levels down too) so I tested for nitrates and I have around 20-40 ppm, does this mean my tank is cycled and I can start doing weekly water changes and add more fish?