What is the acceptable upper limit in nitrate levels for shrimp/fish - around 20ppm no?
Unless you have only mosses, most plants are nitrate hogs. Like someone else mentioned, they eat it up like candy.
I can't see it being anything aside from bad reading or ferts (unless your substrate it leeching nitrates), Try cutting back for a week or two and see what happens.
I dose 45 ppm every week and have for perhaps 100 species of fish, breed them too. Cory's Sturisoma, several other lesser common rare plecos, and catfish, Discus, Angel fish, Apisto's etc.
We do not KNOW what the upper ranges are for NO3 from KNO3.
I've gone up to 160ppm for about 1 month. No issues.
Carmargo et al, 2005 did a decent review of nitrate toxicity, guppies and other warm water species had very high LD50's for NO3. Cold water fish are more sensitive to NO3, since few keep cold water tanks and plants, this cannot and should be used a metric eg, Trout fry are not the same thing.
Still, dosign errors can occur and many have over the last 20 or so years with KNo3...........no one has ever killed fish as far as any confirmed cases I've seen to date with KNO3.
Almost every week someone gasses their fish, and that's just the honets folk who admit to it.
No one seems to give a rat's hind quarter there.
Still, a target of 10-30ppm for the plants is adequate for most all cases, I add about 45ppm, but some of this is removed as the week progresses, so it's likely around 20-30ppm range.
Not that targeting that ppm range is all that difficult.