Sorry to hear. That has to be the best looking jardini I have ever seen. I have seen alot of them. The coloration on it's head and large fins, nice body shape with smooth bronzey color blending at the rear with darker base. No artificial lighting to enhance colors. Very nice, sorry for the loss.
I understand why you put it down. I would have done the same.
If you smell something funny with the water always good to check as the nose knows.
In the future have that test kit ready and use regularly. They have saved me more than a few times over the years.