I had a similar situation after I gravel vacced my well established tank and the conclusion I came to was that I had disturbed the fertilizer tabs (not from the Co-Op) I had placed in the substrate and released ammonia/nitrite into the water column. It was briefly at 10ppm and I had to do 50% water changes for a few days in a row before the I was able to get out in front of it. Had a bacterial bloom as well which I assume was the tank trying to also deal with the excess nitrites. Haven't used the gravel vac since and actually added additional fert tabs but no subsequent spike.
Nitrite spike after water change /vac
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I had a similar situation after I gravel vacced my well established tank and the conclusion I came to was that I had disturbed the fertilizer tabs (not from the Co-Op) I had placed in the substrate and released ammonia/nitrite into the water column. It was briefly at 10ppm and I had to do 50% water changes for a few days in a row before the I was able to get out in front of it. Had a bacterial bloom as well which I assume was the tank trying to also deal with the excess nitrites. Haven't used the gravel vac since and actually added additional fert tabs but no subsequent spike.