Stingray treatment

[lightsout]

Anyone have any idea what this is? I thought it was sand at first but it's been like this for atleast a week or two. I find notice this week he hasnt been eating much at all.

Everyone else in the tank are fine and eating well.

Any tips how to fight this will be appreciated

 

[viper0397]

Looks maybe like something is biting it. Or maybe sucking on its slime coat. What other tank mates do you have?

You can try daily water changes to help and add salt to the aquarium.