Why did my black fin shark turn black?

I have 2 black fin sharks.. one is a light shiny gray and the other is a dark charcoal grayy.. they both have black fins with a white trim and catfish like whiskers being that they are in the catfish family.. i don't know why your fish could have turned black but it may be growing? that would be my best guess.. they grow up to about 18in long and need a 30 gallon to start when they aare purchased at about 2.5in and need to be increased in tank size up to at least a 100 galloon tank when they reach full length or they will streessss out! go to where you bought your sharks and check with them to see what is up with ur fish..