Just do a 100% water change, or a couple 75%, and see what happens. Sounds like the tank is cycled, but there is something in it adding to the bioload. It most likely is from bacteria and other microorganisms dying off, possibly within the rocks (you can take them out and soak them in peroxide or bleach if you want, just make sure you then soak them in some fresh water with dechlorinator in it afterwards). Just stay up on water changes and it will almost certainly stabilize in short order. If it's not big enough to see, it can't be an endless supply of organics. 160 ppm of nitrates is unhealthy for fish and livestock, but it's also still only 1 part in every 6,000+ parts of water. So it's not like you have a rotting fish in there somewhere.