I haven't quite gotten brave enough to do that large of a water change yet. I think the most I've done on that tank is 40% and that was quite the adventure. haha. Though even with doing water changes every other day or every day the level has remained a constant 0.25. The same thing is happening in my 10 gallon and I've done a 90% in that one. So (and please point out any flaws in this logic) since I'm treating both tanks with prime it leads me to think it's showing ammonia and ammonium levels. I've looked into that for some test kits and the site I found shows the total amount that the test is showing and the amount of actual ammonia (with a crazy long complicated equation). I'm going to try to find that page again because if this is the case then it will be useful for at least keeping me from pulling my hair out.