Green neon tetras becoming translucent
As the attached picture shows, I have two (one pictured) green neons that are becoming translucent. Their iridescence has also faded and become mottled.
This started about four weeks ago when I raised the temperature in the tank to 89 degrees to stave off what looked like a possible ich infection on a tank mate. The raised temperature continued for about two and a half weeks. No ich outbreak. I thought these guys would return to normal appearance after the tank was returned to 78 degrees. Now more than one week later, they still look translucent and dull. I have another green neon that appears normal. I have one cardinal tetra that also appears normal.
Other key factors:
1. The fish are behaving normally and have since the problem appeared. No flashing. No isolation. Swimming in normal movements/patterns, especially when compared to the other tetras in the group that have a normal outward appearance.
2. Their physical appearance is otherwise normal. Specifically, they aren't losing body mass and aren't suffering from any apparent skeletal deformities.
3. They are eating normally.
4. They are schooling with the other neons.
5. The problem hasn't appeared to get any better or worse since onset. In other words, the change appears stable.
Anyone experienced a similar issue? Any thoughts on a cause? I'm not overly concerned given the normal behavior and eating. Since it's lingered this long, however, I'm wondering if there is anything I can do to return their former appearance.