Could try adding some more fish the fish may nit be creating enough waste to cycle the tank properly. I had this problem in a 50gallon tank with one angel(as our others died) and we changed our filter and we couldnt get the tank to cycle again so were told to add more fish as the angel wasnt creating enough waste and we added 3 more angels a brisltenose and a kirbensis cichlid and it finally started to cycle and had no more problems with it
Sent from my LG-E460f using Aquarium Advice mobile app