20 gallons!!!!! Good grief, yup I know why the knife died. Dude on a regular stocked tank, 50% a week is normal, on an over stocked tank..... 50% a day might be more appropriate.
Your DOC's, and nitrates are going to be at toxic levels, your pH has probably plummeted and the KH is going to be so low it might not be readable.
With the KH gone, the pH goes bonkers and *poof* fish start dying.
You are going to need to do daily 50% changes for a few days even with out that knife in there to bring the water paramaters back to a healthy state. Then start doing 50% weekly.
DOC's are Dissolved Organic Compounds. They are the summation of all carbon based molecules released by living things from Carbon dioxide to, hormones, pheromones and dead skin cells. They comprise such a huge variety of compounds that there is no one test for all of them, you would have to test for each one independently and add them up to get a total. Excess hormones and pheromones in the water cause all sorts of problems within and between the species sending out and receiving these "chemical messages." Additionally a large number of pathogenic bacteria live and thrive in environments rich in carbon compounds. Traditionally the nitrate test is used as an estimator of DOC's in the water, as they will typically climb together. However using pothos or aquatic plants to reduce the nitrates will mean that the nitrate test is no longer a good estimator of DOC's.