How much air flow and surface movement is there?
How deep is the water?
How's the ammonia? Of the three, this is most important.
Always question everything

If all of these are good, I'd still suggest completely changing the water. Sometimes starting from scratch is the best way to eliminate issues. I know it'll break down your undergravel...you can get some Cycle to help with that. If you've tried everything else, it may be all that's left that you can do.

Maybe pull out the ornaments and replace them one by one once she's better so you can see if that might be it. Even though your other one is good, she might be having a harder time with it...kind of like us with our allergies, everybody's different.

Hope she's back to being Scarlett soon.