I have to give a disclaimer: I'm not discounting the sadness - and oddity - of losing fish you have had a long time, I am just offering a few things to consider.
I was doing a larger than normal water change
That could explain part of the problem. There are just so many variables.
Regardless of how long you have had the fish, take into consideration how long Sheldon has been living with the fish. I would guess they have become accustomed to a turtle/fish environment versus a fish-only environment. If the change is more than normal for Sheldon, then it is more than normal for the fish and can have negative results.
According to a story I found, "...When a sudden, large water change occurs, it causes such a drastic shift in the makeup of the water that the fish often cannot tolerate it and they die. Those that do not die immediately are stressed and may succumb to disease over the next few weeks or months. Naturally the owner thinks that the water change was the cause, and therefore, a bad idea." This is from the following article, but please keep in mind, that Sheldon is part of the mix, so you have to take the story in context, and not focus solely on the fish-only aspects -
http://freshaquarium.about.com/od/water ... gekill.htmAs for Sheldon living in a toxic wasteland, I had the same concerns over Penelope at one point.
A sage on this site gave me some advice "Remember it's a turtle. It can climb out of the water and breathe air"
That was the *smack* I needed. Against my deep seeded desire to snag her and put her in a perfect environment and wanting an instant fix, I let it run it's course and the water health was fine in the end.
I hope things turn out well for you in the end.
Many, if not most, aquarium specialty stores do water tests for free. Maybe take a sample to them and see what they say from the fish perspective.
