you need to stop getting caught up on the fact that some fish are doing fine while others are not. Different species can have much higher tolerances compared to others. So while one fish my look fine you need to focus on the FH that is not doing ok.
there has been studies that show how some fish cannot tolerate 5ppm nitrates and others can tolerate over 100ppm nitrates.
so again, just keep doing water changes. you seem to be reducing the nitrates which is good. a few more water changes and you should be at a decent spot. its going to take a while for the FH to fully recover. Sometimes it can be over a month.
Don't rush it. keep up with every I an others have mentioned and keep a close eye on him, you should see improvements. and by the sounds of it he is already moving around which he wasn't before.
there has been studies that show how some fish cannot tolerate 5ppm nitrates and others can tolerate over 100ppm nitrates.
so again, just keep doing water changes. you seem to be reducing the nitrates which is good. a few more water changes and you should be at a decent spot. its going to take a while for the FH to fully recover. Sometimes it can be over a month.
Don't rush it. keep up with every I an others have mentioned and keep a close eye on him, you should see improvements. and by the sounds of it he is already moving around which he wasn't before.

