Fish's fin-tips turning blackish. Whats causing this?

Question

Only two of my fish (Kribensis) have this happening to them, while my other fish (Zebra Danios) don't have this darkening of their fintips.

The Kribs are behaving normally. They arn't acting as if their fins are irritated. I am not noticing any unusual behavior. The rest of their colors are very vibrant and bright as usual. It's only the very tips of their bottom fins that have a blackish tint to them.

I tested the ammonia, nitrate and nitrite and they all read next to zero. I'm doing a 25% water change anyway. I do so weekly.

Is there anything else that could cause this darkening besides ammonia burns?

Answer 1

I think this is something completely normal

my kribs colors look the same way

nothing to worry about, since you have good water conditions