Once you get the nitrate tester, a couple of things to know. One is follow those directions perfectly, I learned this the hard way, and the first couple of times you test, do it twice and make sure you get the same results each time. Lastly, Assuming you have no plants, nitrate is a great test to determine maintenance effectiveness. I would reccomend trying to get the level down to 5-10 ppm untill Gulp gets better and then keep it steady under 20 ppm from there on out ( this is my personal standard for Oscars in particular due to hith). This is not an easy feat with an O but with your nice big tank, and you low bio-load, once you get the numbers down maintaining them will be pretty easy. Another thing to bear in mind is that your filter cartridges trap the waste, but it is still part of the system so more frequent rinsing will help immensely in the reduction of nitrates. Just remember to be nice to your bacteria so they don't all die at once.