Hey gang. My daughter has a 36g cycled tank. It's housing 6 guppies and 5 sterbaI cories. They have all been in the tank for over a month now. I have been testing the water once a week and doing weekly water changes by vacuuming the gravel. I did one last night. Checked the water today and my nitrates were 20-40ppm. I have never seen them this high ever. I feed once a day. The fish all act fine. Just wondering what could be causing the spike. She doesn't have any live plants or anything to consume some of the nitrates but water changes. Any thoughts on what could be causing this?