Hey everyone, I have two clownfish (juveniles) in my 20 gal tank with an HOB and circulation pump. All the water parameters are fine but I recently noticed the bigger clown of the two has these brown spots around the gills. He also is much less active than my other smaller clown and usually sticks around one area but still eats, although he is not as excited for food. Their poop is kinda stringy although im not sure if that's because i fed them too much Mysis shrimp accidentally. Ever since I got him 2 weeks ago I can't remember if he's had the brownspot on him when i did get him or if it slowly developed. I've done a lot of googling and can't find any answers. I've attached some photos for you guys to take a look at.
I need help identifying what it is especially if it's a disease and what i can buy to cure it/the steps to take. Thank you! would really appreciate your knowledge
Here's my smaller healthier-looking clown - no brown spot
Here's my slightly bigger unhealthy-looking clown - brown spot