Mistakes like that happen. Glad your tank is OK as far as ammonia. I do have a question though, you say nitrates are also zero? maybe you meant nitrites?.
One important note on nitrate measurement:
No matter the brand, there's only one test for nitrates (same in freshwater and saltwater) it has 4 reagents, one of them is solid (zinc dust).
Some manufacturers, as API, include this solid particles in the reagent bottle #2. Others as Tetra and Marine Enterprise, keep it separate in a vial. Shaking vigorously is a must for accurate readings: false negatives can happen if these are not properly mixed.
I use test strips that measure nitrites and nitrates as screening testing. Strips will likely give false positives for nitrates although not likely for nitrites; in nitrates readings with strips you can easily end up with an error of measurement of 20ppm or more above actual values.
Don't worry too much. Your Oscar is sulking. As with us humans, this shall pass. Keep your active nurturance toward him/her and things will get back on track.
When I removed a few fry from my pair they were really mad at me. They see me as a friendly person again.
Pepe
Santo Domingo