Sexy Shrimp: How High Is High No3?

[Caine]

All the info I've been able to find about Sexy Shrimp is that they do not tollerate high NO3 levels. My question is "How high is high?" for these shrimps?

I have a little 10x10x10" invert tank that seems to be doing well so far. It has only one little frogspawn. Its pretty happy and it has grown up a bit from when I first got it. I'm feeding it weekly with a little chunk of shrimp on Thursdays followed with a water change on Saturday. NO3 levels are 20ppm just before I do my water change which resets it down to about 5 or 10 ppm or so (I'm not good at reading the colours).

Are those safe levels for sexy shrimp? If no, how high is still safe for them?

Thanks!