High nitrate planted tank help!

[FishingMouse]

I have a planted 10 gal an during my entire cycle the plants kept the nitrates

 

[FishingMouse]

Stupid iPhone! Anyway, the entire cycle my plants kept the nitrates below 10 ppm. I ran it for a couple days after ammo and trites went to zero, adding ammo and making sure they converted all the way in 24 hours.....so I did a big wc and added my fish the next day, 4 dainty cories and 3 tiger nerites to help with my diatom problem.
I noticed clamped fins, so tested, got shockingly high nitrates, did the test again making sure I made no mistakes and got the same results. I'm rectifying it now with a couple back-to-back wc, but I was wondering if I could get some input on what could have caused this? I had been dosing ammo to about 4 ppm before letting it zero and adding fish......and I even have more plants than when I started my cycle, so I'm really confused as to how this spiked so high in 3 days!

 

[aqua_chem]

What's "shockingly high" to you?

 

[FishingMouse]

160 ppm

 

[Sillyfishies]

Or higher. On the test it'll just say 160. But it could be 1000!

 

[FishingMouse]

I just tested and I got it down to about 80, did another wc and will test again in an hour ><

 

[FishingMouse]

Not tested yet, but quick update; the Cory cats are back to their crazy antics after just two wc! Would still be interested in opinions regarding why this happened so fast, especially in a planted tank where the plants had been gobbling up the nitrates before.....