What the spike is doing to your fish
Mid-cycle, the bacteria that consume ammonia have usually established before the slower colony that consumes nitrite, so nitrite rises in a tank that looked nearly finished. It matters because nitrite crosses the gills and interferes with the blood's ability to carry oxygen — fish can be surrounded by water and still suffocate slowly. The early picture is easy to miss: fish a little quieter, appetites off, gills working harder than the temperature explains, the bold ones hanging nearer the surface or the filter outflow where oxygen runs richest. By the time a fish is gasping at the surface or lying on the gravel, it has been struggling for a while. Treat any readable nitrite result with fish in the tank as the day's main job.
Dilute, dilute: the water-change rhythm
There is no product-free shortcut around the arithmetic: the nitrite is in the water, so the water has to leave. Change a generous portion — a third to a half — with dechlorinated water matched closely to tank temperature, and test again after it settles. Repeat daily, and twice daily if the reading stays stubbornly high, because the bacteria are still growing toward the problem and your changes are the bridge that keeps the fish alive until the colony catches up. Make up change water carefully during a spike; a sloppy change that adds chlorine or swings the temperature trades one stress for another. Keep the removed water for nothing else, top the tank back gently, and write down each reading and each change so you can see the trend bending.
Food, air and the things not to touch
While nitrite is up, the tank's inputs must shrink. Stop feeding entirely for a day or two, then return with small meals — every mouthful becomes waste that becomes ammonia that becomes more nitrite, and hungry fish are in no danger over the short stretch that starving the spike requires. Add aeration if you can: an airstone, a raised outflow, anything that agitates the surface, because oxygen-rich water is what the fish's compromised blood needs most. Just as firmly, leave the filter alone — no rinsing media, no replacing cartridges, however grubby they look. The colony you are waiting for is growing on those surfaces, and cleaning now restarts the clock the fish cannot afford.
Knowing when this stops being a home fix
Most fish-in cycles resolve: readings soften over days, appetites return, and the changes can space out. Escalate when the pattern breaks the other way — a fish dies, several fish gasp at the surface despite fresh water and aeration, or the reading will not fall at all across repeated changes. Those are signs the problem may be bigger than the cycle, and an aquatic vet or an experienced specialist shop should see the readings and the fish. Hourly decisions blur together during a spike, so keep the written readings close at hand for the moment you brief a vet or a specialist shop about what the tank has done. Put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.