What is this White line at base of fin?

[KathyComeau]

Hello,
I have three Fire Mouth Ciclids that I’ve had for several months. Nothing new has been introduced in the tank. One of them has developed a white line along the base of its dorsal fin.

 

[PheonixKingZ]

Hello and welcome to the forum!

Actually looks like fungus, or velvet. Both of which are caused by poor water quality.

Some other members may be able to confirm...