May need to add a fish. I’ve got 2 in my 15 gallon and I don’t dose anything but my phosphates stay about .08 and nitrates around 10 with weekly 10-20% water changes. Also I would recommend picking up the Hannah phosphate ppb and high range nitrate tests at some point.
I have all salifert tests and was also having the same problem with phosphates. Looked crystal clear like 0 to me. Once I used Hannah, I am getting a much more accurate test. If you can afford it, the high range nitrate test is also excellent and accurate. Other than those 2 I love the salifert for calcium, magnesium and alkalinity.
sorry to hear about your Duncan.