Nitrate spike?

[GreenSpottedPuffer]

Could a dead fish that was in my tank for no more than about 3-4 hours max (more likely 2) have caused a nitrate spike? After my puffer died yesterday, I thought I should check the nitrates today...10!

I can't imagine why else they would have spiked up to 10 in the past few days but I also thought it took much longer than that for the fish to start decaying and causing water issues. I tested last about four days ago and they were 0 as they have been for months.

The only reason I am asking is because I want to figure out if this is from a dying fish or if I have a nitrate problem

I have also added some new fish but they are small and I have no increased feeding even yet.