i'm not sure what to make of this, because a couple of days ago i bought this meter and placed in the sump it was absolutely yellow . But then I wanted to test it as I saw video on that topic so I took it out and exposed it to an open bottle of glass cleaner and the meter immediately darkened. to blue or black . I'm like "oh it works" and I put it back in the sump. That was 48 hours ago and I'm not sure if it's never gone back yellow or if now i have ammonia in the tank. I'm not sure why my ammonia would go up in two days. I just water changed yesterday; but I did add a couple of small blue damsels .