Nitrite Spike?

[This Old Spouse]

I had a couple of fish acting strangely today, and even though I did a 25% water change earlier in this 29g tank, I thought I should check the levels. Well, the ammonia was 0 but the nitrite was elevated! And I don't know by how much because I couldn't find the color chart to save my life. I'm thinking it was probably at about .5. What would cause this?