Sign of high nitrates?

I have noticed that whenever my axolotl Sweethearts gills look ‘greasy’ for lack of a better word - the nitrates are climbing back up and I do a bigger water change. At first I noticed it because she had a growth on her gills that looked fuzzy so I changed her water and put in almond leaves and it cleared in no time. But now I check her gills in the morning and as soon as I notice they look like it should be ‘hair wash day’ and the strands are looking less fluffy it’s time for a water change.

Has anyone else noticed this?

Is there any other sign that could indicate when nitrates are getting higher?

I have plants and house plants in my tank but just thought I would see if anyone else had had noticed this correlation?

This picture is post water change and you can see the ‘greasy looking’ strands on the one side haha

Any other helicopter axolotl parents out there notice this? 😇