Each api test has a time you must wait before reading it. Some will degrade after that time and turn all sorts of weird. Like that one turns a deep yellow after a few hours, then another day later it will leave residue in the test vial and mess up the next test unless you scrub the inside. For the ammonia test, typically if itís near zero itís the color youíll looking at, not the yellow on the sheet but yellowish. The sheets yellow doesnít match very well. However once you get into the green it will match pretty easily. API mentions looking up through the bottom to see if there is any hint of green, which helps but most likely if itís that hard to tell your fine. Thatís my 2 cents.
Yep,that's why the kits come with directions, and time for reactions/readings.
Just as likely the tester, rather than the test kit at fault.
Hold your finger over the vial instead of capping it, and skin cells /oils, might give wonky reading.
Too much water,too much solution,Ditto.
Shake the bottle too long ,or not long enough,Maybe true reading,maybe not.
Let the vial set too long, or not long enough.(why?)
Residues from previous test's still in the vial (clean em).
Inability of some to follow direction of any kind.
I am suspicious of test kit's as well as tester's.