I have not had a problem I thought it was but did tests on my kids tank and it was correct. I have used other test kits this was a few years ago though I do perfer API as the test intergers are closer together some of the others which makes to easer to make a read as to what the true reading is. Once your tank has settled down it whould read 0 or neer 0 for the ammonia, nitrite and nitrate all the time and if you only add 1 fish at a time you should not have large spikes in ammoinia, nitrite and nitrate. Their marine test kit I have not use I am going to be buying the CA test kit today, i'll let you know what I think about it.

Bill