Is this gill curl?

[JKSchnoo]

Have you tested your water?

Yes

If yes, what is your ammonia?

0

If yes, what is your nitrite?

0

If yes, what is your nitrate?

80-100

If I did not test my water...

...I recognize that I will likely be asked to do a test, and that water tests are critical for solving freshwater health problems.

Do you do water changes?

Yes

What percentage of water do you change?

21-30%

How frequently do you change your water?

Every week

If I do not change my water...

...I recognize that I will likely be recommended to do a water change, and water changes are critical for preventing future freshwater health problems.

Hi Everyone-

Noticed the gills on my two red tail cats are starting to slightly curl outward. It comes and goes though, doesn't flare out all the time. They are in my 500 gallon. 0 ammonia, 0 nitrite, nitrates I battle, but usually stay around 80-100 ppm. Pictures below.

 

[duanes]

Looks like it to me, and especially when I see your nitrate levels of 80 to 100 ppm.
These and other fish from the Amazon region, come from waters where nitrate is almost undetectable.
To me you should up your water change schedule to at least 100% per week.
Doesn't need to be 100% all in 1 day, but broken up during the week to equal 100 %

 

[Fishman Dave]

Not entirely sure TBH. That redtail does look to have had slight damage previously on its left pectoral and you may find from the side or front view that this fin is slightly raised compared to its right one. It could be the positioning of the pectoral pushing out the left gill cover slightly which also may explain why you see the curling coming and going a little.
Obviously more water changes are always good with higher nitrates, but you may also find if the above is the case that sorting the nitrates, whilst making for a better environment, doesn’t fix the issue.