black spot on betta's tail fin

[baker]

I've got a 7-gallon tank, heated and filtered with low flow. It's heavily planted and I've only got a couple of nerites and some cherry shrimp as tank mates. My water parameters are good - pH is 7.2, ammonia and nitrates are 0. I've also got almond leaves and alder cones in the tank to add some tannins.

I've noticed the end of my betta's tail fin is turning black. He's about 4 years old, so I'm wondering if this is just a normal part of aging. I know it can be a sign of fin rot, but my water parameters are solid and there's no fin degradation. I did a 40% water change about 5 days ago, but it didn't make a difference - the black hasn't spread, but it hasn't gone away either. Any insight would be great, thanks.

[swim_deep]

I'm thinking maybe the shrimp, or possibly even the nerites, could've nipped at your betta's tail fin, causing the blackening. It's possible this is the start of fin rot, despite your good water parameters.