At Day 19 it would make sense to me that you are somewhere in the midst of your nitrite spike (the second phase of three) which often goes on quite a bit longer than the initial phase when you are waiting for ammonia to drop the first time.
To me, backgrounds rarely seem to go with the clean look of the Edge (although art is tricky and you never know what might work) -- many us have had success with plain black backgrounds (mine is mostly blocked from view by plants and the substrate is also black, so it's mostly the fish and plants that catch the eye.)
~~waterdrop~~