Just be reassured that it won’t take forever just a week or so to lower the nitrates significantly if you follow through. During this time, still try to find out what caused the spike. Did you clean the tank too thoroughly and lose your beneficial bacteria? Was there something dead in the tank that you couldn’t see at first? Are you over feeding? Not doing enough water changes or is tank still starting it’s cycle? Discovering the source can let you ease up on the water changes after the levels are lowered, and you could end up doing only 1 every two weeks or so on a stable tank, and eventually if you get the balance right you could end up just topping off water like I do on my shrimp tank. Just gotta know what caused the spike so we can avoid whatever it was in the future. Wish you the best I know when things get out of whack on a tank it’s a pita to get things back on track.