Dark Mark on/in guppy tail

[Tank Mate]

A few months ago we noticed a small dark mark in our male guppy's tail. We kept a close eye on it and since then it's grow a bit as shown in the photos.

Has anyone got any idea what this could be? I've seen a similar post about this elsewhere but there was no conclusion. The fish seems to be behaving and eating as normal.

Apologies in advance for the poor photos - he really doesn't like staying still!

 

[Delapool]

I assume water chemistry is all fine? I'd suspect natural for that location, if on the body I have heard of tumours causing a colour change.

 

[Tank Mate]

Water chemistry is all fine - only concern would be nitrates of 40-60 as that's the content of our tap water.

All other fish completely fine. I'll also note that light doesn't shine through this mark and it stays as a shadow.

 

[Delapool]

I'd think natural unless there is some other sign - clamped fins, etc.

Only issue might be if nitrates are high in tap water, is there a risk of heavy metals?