API is notorious for being off for Ammonia. Other than that it is a reliable test kit. All test kits we use are hobby grade. The main thing we should be looking for are trends and stability. With these things we can figure out what we are doing wrong and be able to figure out if we are doing the correct thing to fix the problems. The OP needs to figure out a better export method. Large fish in a small tank are going to have high NO3 and PO4. They eat and poop a lot. Heavy in and Heavy out are imperative in this situation. I'd use a roller mat, large fuge, and a big skimmer as well as carbon dosing.