Is this a disease or just his markings?

[Brinn2397]

My gold algae eater is doing really well but I’ve been noticing black marks on his back, I’m not sure if it’s normal because he’s acting normal

 

[FishGuest5123]

What are your water parameters? It could be ammonia poisoning.

 

[Colin_T]

it just looks like dark pigment on its body. Chinese Algae Eaters are normally a mottled brown in colour, but they have developed gold, albino, pink and a few other colours. Sometimes the brown comes through on gold or pink fish. Monitor the fish over the next week, if it gets worse then post more pictures but I wouldn't worry too much.

Check your water quality for ammonia, nitrite, nitrate & pH. If there's any ammonia or nitrite then do a 75% water change each day until its back to 0.