Ugh, so I'm in the middle of a Nitrite spike as well. I changed all my gravel Monday evening, did a 50% water change, and all seemed well. Tuesday morning all the fish were gasping for air at the surface and nitrites were at least 5ppm. Strangely ammonia was and is still at 0. I added a 5x dose of Prime + aquarium salt and things improved significantly. Last night the nitrites were still very high, so I did a 65% water change and added nitrifying bacteria. Now it's down to 0.2ppm. Not sure if the reduction is due primarily to the Prime or the water change, so I'll plan to continue daily water changes until things stabilize.