Sounds like you are a knowledgeable hobbyist.
I see no obvious red flags yet. The nitrates are a bit high, esp. for such a light load of one 1" fish. I'd keep them under 10 ppm, better yet under 5 ppm - nitrates in all major rivers are undetectable by tests such as our crude home tests. In standing waters subject to agricultural run offs, nitrates from fertilizer can be quite high.
Nitrates have been implicated in digestion issues in a goldfish study but they have been much higher, ~10x, in that study, and the digestion problems were serious. One must note that measuring nitrates is quite tricky, as in not reliable, esp. the results cannot be trusted on absolute scale and should be taken with a grain of salt even on relative scale.
Hence, AFAIU, our pets may have nitrate-driven digestion issues while the nitrate test shows normal or not-too-alarming readings.
What's your wc routine? I'd make several large ones to bring the nitrates to 5 ppm or below and try to keep them there, that is, unless your tap / source water already has 10-20 ppm.
How long has the fish been in the qt tank? In your possession? Where I leading with this is that this is a qt tank and hence it may have a diverse culture of pathogens, bacterial, viral, or parasitic, how ever latent or active. What's the history of that qt tank prior to the last occupant arriving - was anybody ill in there, died?
Has it fed with gusto before supposedly falling ill or was it acting hesitant, not hungry, etc.?
If you are on municipal water, I hope you don't forget to add dechlorinator. We all can forget.
Hardness is no biggie for highly adaptable synos, goldfish but it needs to be stable. Large swings in hardness, pH, temp., etc. stress fish and can shock them. Swings in hardness and pH occur usually in tanks fed with RO water and having poor buffering ability.
Questions are welcome.
