Is it possible for the test liquids to differ from batch to batch? 3 weeks ago i bought a new batch of nitrate test liquid, my old batch had run out. The expiry date on my new batch is 2019. The expiry date on my old batch was 2017 so they are definitely different batches. I do my water change when my nitrates start creeping into the deep orange colour, maybe 20-30ppm. This is every 6 days according to my old batch of test liquid. However, my new batch of test liquid is showing my nitrates to be only a mid orange colour after 6 days. According to my new test liquid i could probably stretch to every 7 days now. Nothing has changed in my tank feeding and stock wise. Has anyone else noticed similar anomolies when starting a new batch of test liquid, whether it be nitrate or any of the other water parameters?