Also, when you get a chance, take a tap water sample, let it sit out in a closed container overnight. Test in the morning and take a pic of the results above the color card. Take a test of the tank water and take a pic above the card as well. Post those. Helps to know exactly what we're dealing with. Sorry for all the odd requests, just helps to narrow it down and cancel things out
Also, you may have said it already and I may have missed it, but what kind of food are you feeding (everything, treats, staple food, live, whatever. Also how often and how much). Food generally is the cause of higher trates when the tank isn't overstocked, and clearly the tank isn't. I have more bioload in my 20 long right now! All your fish appear to be quite young